How Many? A Dictionary of Units of Measurement
© Russ Rowlett and the University of North Carolina at Chapel Hill

Table of Contents
About the Dictionary
Using the Dictionary
M

M [1]
an informal abbreviation for million in expressions such as "$500M" for 500 million dollars or "Unemployment Reaches 4M" in a newspaper headline. In binary contexts such as computer memory, M often represents 220 = 1 048 576 (see mebi-, below).
M [2]
the Roman numeral 1000, sometimes used in symbols to indicate a thousand, as in Mcf, a traditional symbol for 1000 cubic feet. Given the widespread use of M to mean one million, this older use of M to mean one thousand is very confusing and should be scrapped.
M [3]
the symbol for "molar" in chemistry (see below).
Ma
a symbol for one million years, often used in astronomy and geology. The "a" stands for the Latin annum.
mab
symbol for "meters above bottom" (bottom of the sea), a unit used in oceanography.
mace
a traditional Chinese unit for weighing precious metals, especially silver. In the European colonial period, the mace was considered equal to 0.1 tael or liang; this would be 2/15 ounce or about 3.78 grams.
Mach or mach (M or Ma)
a measure of relative velocity, used to express the speed of an aircraft relative to the speed of sound. The name of the unit is often placed before the measurement. Thus "Mach 1.0" is the speed of sound, "Mach 2.0" is twice the speed of sound, and so on. (The actual speed of sound varies, depending on the density and temperature of the atmosphere. At 0 °C and a pressure of 1 atmosphere the speed of sound is about 1088 ft/s, 331.6 m/s, or 741.8 mi/h). The mach speed is important to the control of an aircraft, especially at speeds close to or exceeding Mach 1.0. The unit is named for the Austrian physicist Ernst Mach (1838-1916).
maf or Maf
a symbol for one million acre feet. This symbol, commonly used in reservoir management in the U.S., should be written Maf. 1 Maf = about 1.2335 billion (109) cubic meters.
magnitude (mag) [1]
a unit traditionally used in astronomy to express the apparent brightness of stars, planets, and other objects in the sky. For centuries, the brightest stars were said to be of the "first magnitude," with fainter ones of the "second magnitude" and so on down to "sixth magnitude" for the faintest stars visible to the unaided eye. When it became possible to measure stellar brightnesses precisely, it was discovered that stars of a given traditional magnitude were roughly 2.5 times brighter than stars of the next magnitude. Astronomers agreed to define the magnitude scale so that a difference of exactly 5.0 mag corresponds to a brightness difference of exactly 100 times. A difference of 1.0 mag then corresponds to a brightness difference of the fifth root of 100 or about 2.512 times. The scale is upside down: brighter stars have lower, not higher magnitudes, in keeping with the historical origin of the scale. The zero point (0.0 mag) is set arbitrarily so that the stars historically listed as "first magnitude" have magnitude measurements of 1.5 mag or brighter. The brightest stars and planets have negative magnitudes on this scale. Note: the scale is commonly used to describe the apparent magnitude of objects as we view them on Earth, but astronomers also use it for absolute magnitude, which is the magnitude the object would have if it were placed at a standard distance of 10 parsecs (32.61 light years) from earth.
magnitude (mag) [2]
a unit used in earth science to measure the intensity of earthquakes. Geologists actually use several scales to measure earthquake intensity, but the one best known to the public is the Richter magnitude scale, developed in 1935 by Charles F. Richter (1900-1985) of the California Institute of Technology. The Richter magnitude is computed from the measured amplitude and frequency of the earthquake's shock waves received by a seismograph, adjusted to account for the distance between the observing station and the epicenter of the earthquake. An increase of 1.0 in the Richter magnitude corresponds to an increase of 10 times in the amplitude of the waves and to an increase of about 31 times more energy released by the quake. The most powerful earthquakes recorded so far had magnitudes of about 8.5. The Richter magnitude measures the intensity of the earthquake itself, not the intensity of the earthquake's effects: the effects also depend on the depth of the earthquake, the geology of the area around the epicenter, and many other factors. Earthquake effects are rated using the Mercalli scale (see below).
magnum
a traditional unit of volume for wine, generally equal to 2 bottles. This is now exactly 1.5 liters (about 2.114 U.S. quarts).
mahnd
a traditional Arab weight unit equal to about 2.04 pounds or 925 grams.
mål
a Norwegian word for "measure," mål has been used as a name for various traditional Norwegian units. As a land measure, the mål is currently defined to be the same as the dekare, that is, exactly 1000 square meters (0.1 hectare or 0.2471 acre). The mål has also been used as a unit of volume equal to the dekaliter (10 liters).
mandel
a traditional German unit of quantity equal to 15.
man hour
a common unit of labor equal to the work of one person for one hour. The name person hour is increasingly used for this unit.
manpower
an informal unit of power equal to 0.1 horsepower or about 74.57 watts. The unit seems to have been invented by American engineers.
manzana
a traditional unit of land area in Central America. The manzana is the area of a square 100 varas on a side; it thus varies according to the length of the vara. The Costa Rican manzana equals 0.698 896 hectares or about 1.727 acres. Very similar units are used in Guatemala, Honduras, and Nicaragua. The word manzana means an apple, but the unit is probably related to manzanar, orchard.
marathon
a traditional unit of distance used in athletics. The length of a marathon is exactly 42 195 meters (about half an inch longer than 26 miles 385 yards). Invented for the first modern Olympic Games at Athens in 1896, the marathon recalls a run made in 490 BC by a Greek soldier (possibly Pheidippides) to bring to Athens the news of the Greek victory over the Persians at the Battle of Marathon. However, the actual distance from Marathon to Athens is only about 36.75 kilometers. The 1896 run was exactly 40 kilometers from the Marathon Bridge to the Olympic Stadium. At the 1908 Olympics in London, a course of 26 miles 385 yards brought runners from Windsor Castle to White City Stadium (the story is that exactly 26 miles was intended, but Queen Alexandra insisted that the finish line be moved in front of the royal box). The marathons at the Olympic Games varied in length until the 1924 Olympics in Paris, when the International Olympic Committee adopted the 1908 London distance as official.
marc, marco, or mark
traditional units of weight in various countries of Western Europe. In each country the unit equals 1/2 the unit corresponding to the English pound. Thus the French marc equals 1/2 livre, 8 onces or about 244.75 grams; the Spanish marco equals 1/2 libra or about 230 grams; the German mark equals 1/2 pfund or about 280.5 grams; and the English mark equals 8 ounces or 226.8 grams. The English unit was used almost entirely for measuring precious metals.
marine league
an informal name for the league as used at sea: a unit of distance generally equal to 3 nautical miles (5556 meters).
mark twain
see twain.
marla
a traditional unit of area in Pakistan. The marla was standardized under British rule to be equal to the square rod [1], that is, 272.25 square feet, 30.25 square yards, or 25.2929 square meters.
mas
symbol for milliarcsecond, a unit of angular measure commonly used in astronomy.
masha
a traditional unit of mass in India and Pakistan, standardized under British rule as 15 grains, 1/12 tola, or about 0.972 gram. In Pakistan, the unit is still used sometimes for the weight of precious metals.
masl
a common symbol for "meters above sea level" used in geology and geography.
maß (mass)
a unit of volume for beer in Germany and Austria, usually equal to one liter today. The traditional Bavarian maß was about 1.07 liters.
mAU
a symbol for the milli-absorbance unit. An increase in absorbance of 1 mAU corresponds to a reduction in transmittance of about 0.2305%.
maund
a traditional unit of weight in India and throughout South Asia. The maund varied considerably, but during the period of British rule in India it was standardized at about 82.286 pounds or 37.3242 kilograms. The maund is divided into 40 seers. Since 1980, Pakistan has used a metric version of the maund equal to exactly 40 kilograms (88.185 pounds), thus making the seer equal to the kilogram.
maxwell (Mx)
a CGS unit of magnetic flux, equal to 10-8 weber. In a magnetic field of strength one gauss, one maxwell is the total flux across a surface of one square centimeter perpendicular to the field. This unit was formerly called the line [2]. The newer name honors the British physicist James Clerk Maxwell (1831-1879), who presented the unified theory of electromagnetism in 1864.
MBF or MBM
traditional symbols for 1000 (not one million) board feet, a unit of volume for timber equal to 250/3 = 83.333 cubic feet or 2.360 cubic meters. "BM" stands for "board measure."
mbsl
a common symbol for "meters below sea level" used in geology and oceanography.
MBH, MBtuh
symbols for 1000 (not one million) Btu (British thermal units) per hour, a unit traditionally used in the U.S. heating and air conditioning industry to state rates of heating or cooling. One MBH equals about 0.293 071 kilowatt.
mc- or mc [1]
alternate symbol for micro- (see below). This prefix is often seen in the symbol mcg for the microgram. The use of the symbol mc- for micro- became established because typewriters and early dot matrix printers did not have the proper symbol µ-. Also, many hospitals require the use of mc- in all handwritten notes and records, because a hastily written "µ" may be mistaken for an "m," leading to serious dosing errors. However, the use of mc- for micro- is also confusing and should be avoided as much as possible. One source of confusion is that mc- may be interpreted as "millicenti-", a mistake that could also lead to dangerous dosing errors. Occasionally, mc has been used as a symbol for the micron (see below).
mc [2]
Italian abbreviation for the cubic meter (metro cubico). This is a non-standard symbol; the proper symbol is m3.
MCF
a traditional symbol for 1000 (not one million) cubic feet, a unit of volume equal to about 28.317 cubic meters.
Mcfd
a symbol for 1000 (not one million) cubic feet per day, a unit of water flow used by many U.S. water supply companies and agencies. 1 Mcfd = 19.665 liters (5.195 U.S. gallons) per minute.
Mcfe
a symbol used in the natural gas industry for 1000 (not one million) cubic feet of gas equivalent (cfe). This is really an energy unit equal to about 1.091 gigajoules (GJ).
mcg
an alternate symbol for the microgram. Although the SI symbol µg is preferable in print, the symbol mcg is used widely in medicine, and its use is required in many hospitals and clinics. This is because a handwritten µg is too easily mistaken for the milligram symbol mg, possibly leading to serious medical errors.
mcL
a symbol sometimes used for the microliter (µL).
MCM
a symbol for 1000 circular mils, a unit of area equal to about 0.5067 square millimeter commonly used in stating wire gauges. This symbol is being replaced by the less-confusing symbol kcmil.
Mcps
a symbol used for 1000 (not one million) centipoises, a unit of dynamic viscosity. This is a jarring addition of an obsolete English prefix to a metric unit, and its use risks a major misunderstanding since M- is the metric prefix for a million rather than a thousand. 1 Mcps equals 10 poises, so the proper name of the unit is decapoise (daP).
MCU
a symbol for milk clotting unit, used for measuring dosage of bromelain, an enzyme used as a digestive aid and for reduction of pain and inflammation. This unit cannot be converted to a weight unit, because different preparations of the enzyme differ in activity. Bromelain is also measured in gelatin digesting units (GDU); 1 MCU equals approximately 2/3 GDU.
mease
a unit of quantity formerly used by fishermen. The mease equals the number of herring in a basket, roughly 620.
measure
a musical unit representing a series of beats [2] (rhythmic stresses) with one primary or accented stress. A measure is also called a bar because the end of a measure is represented in musical notation by a vertical bar.
measurement ton (MTON or MT)
a unit of volume used for measuring the cargo of a ship, truck, train, or other freight carrier, equal to exactly 40 cubic feet, or approximately 1.1326 cubic meters. This unit was traditionally called a freight ton (see ton [5]), but that term now means a metric ton of freight in most international usage. However, the confusion seems impossible to dispel; some shippers are now using "measurement ton" to mean a metric ton of freight. (The way out of this dilemma is simple: measure volume in cubic meters and weight in metric tons.)
mebi- (Mi-)
a binary prefix meaning 220 = 1 048 576. This prefix, adopted by the International Electrotechnical Commission in 1998, is intended to replace mega- for binary applications in computer science. (This replacement does not seem to be happening.) The prefix is a contraction of "megabinary."
MED
a common symbol for "minimum erythemal dose," the smallest amount of ultraviolet radiation that produces observable reddening (erythema) of the skin. (Skin is sensitive to reddening by radiation in only a narrow band of wavelengths around 300 nanometers.) The MED obviously varies from one person to another. Doctors and tanning salon operators typically use a value of 200 joules per square meter (J/m2), which represents the MED of a highly sensitive individual; persons with dark skin have MED's in the range of 1000 J/m2. Regulatory agencies are moving to use of the standard erythemal dose (SED), a unit equal to exactly 100 J/m2. In tanning, a dose rate of one MED per hour is equivalent to 55.55 milliwatts per square meter of skin surface.
meg
informal contraction of "megabyte," used in computer science.
mega- (M-) [1]
a metric prefix meaning 106, or one million. (The form meg- is often used before a vowel, as in megohm for one million ohms.) The prefix is also common in ordinary language, meaning "very large," as in megabucks or megadose. The prefix is derived from the Greek word for large, megas.
mega- (M-) [2]
in measuring the storage capacity of a computer, the prefix mega- often means 220 = 1 048 576 instead of an even one million. By a 1998 resolution of the International Electrotechnical Commission, the new prefix mebi- (Mi-) is supposed to replace mega- for 220.
megabar (Mbar)
a metric unit of pressure. The megabar equals one million bars, 100 gigapascals (GPa) or about 14.503 million pounds per square inch. Such intense pressures are found inside the earth or in various advanced scientific experiments.
megabarrel (Mbbl, Mbo, MMb, or Mb)
a unit of volume used in the energy industry, equal to one million barrels of oil. One megabarrel equals 42 million U. S. gallons, which is about 158.987 megaliters (ML).
megabase (Mb)
a unit of genetic information equal to the information carried by 1 million pairs of the base units in the double-helix of DNA; also used as a unit of relative distance equal to the length of a strand of DNA containing 1 million base pairs. In humans, one megabase corresponds approximately to a gene separation of one centimorgan.
megabecquerel (MBq)
a unit of radioactivity equal to one million atomic disintegrations per second or 27.027 microcuries.
megabyte (MB)
this unit of information is very common in the computer world, but it is poorly defined. Often it means 1 000 000 bytes, but sometimes it means 220 = 1 048 576 bytes. As if that weren't confusing enough, the 1.44 megabytes stored on "high density" floppy disks are actually megabytes of 1 024 000 bytes each. This uncertainty is a major reason for the recent decision of the International Electrotechnical Commission to establish new binary prefixes for computer science.
megacycle (Mc)
1 million cycles, a term sometimes used as an informal name for the megahertz.
megacycle per second (Mc/s)
an older name for the megahertz.
megadalton (MDa)
a unit of mass equal to one million atomic mass units. See dalton.
megaflops (Mflops)
a unit of computing power equal to one million floating point operations per second. See flops.
megagram (Mg)
an SI unit of mass equal to one million grams or 1000 kg. This means the megagram is identical to the tonne (metric ton). Large masses are almost always stated in tonnes in commercial applications, but megagrams are often used in scientific contexts. One megagram equals about 2204.623 pounds.
megahertz (MHz)
a common unit of frequency equal to one million per second. Frequencies of radio waves are commonly stated in megahertz.
megajoule (MJ)
a common metric unit of work or energy. The megajoule equals one million joules, which is approximately 737 562 foot pounds, 947.8170 Btu, 238.846 (kilogram) Calories, or 0.277 778 kilowatt hours.
megakelvin (MK)
a unit of temperature equal to one million kelvins. This unit is used in astrophysics; temperatures in megakelvins are found in the interiors of stars or in highly excited plasmas. The reciprocal megakelvin (MK-1) is used in colorimetry.
megalerg
a CGS unit of energy equal to 106 ergs or 0.1 joule (0.073 756 foot pound). The "l" was added to "mega-erg" to make the unit pronounceable.
megaline
a metric unit of magnetic flux, equal to one million lines [2] or 0.01 weber.
megaliter (Ml or ML)
a metric unit of volume equal to 1000 cubic meters. Commonly used in reservoir and water system management outside the U.S., the megaliter equals 264 172 U.S. gallons or 0.810 713 acre foot.
megalithic yard
a unit of distance equal to about 83 centimeters or 2.72 feet, defined in 1951 by the Scottish engineer Alexander Thom (1894-1985). Thom claimed this unit was used in the construction of many megalithic monuments, including Stonehenge. If so, the unit was probably measured by the length of a workman's arm.
megameter (Mm)
a metric unit of distance equal to 1000 kilometers or about 621.371 miles. Although this appears to be an appropriate unit for longer distances on the earth, the megameter is seldom used.
megampere (MA)
a unit of electric current equal to one million amperes. This unit is used in plasma physics and fusion research.
meganewton (MN)
a metric unit of force equal to one million newtons. One meganewton equals about 101 972 kilograms of force or 224 809 pounds of force. The main engines of the U.S. space shuttle have a maximum thrust of about 2.28 meganewtons.
megaohm (megohm)
a common unit of electric resistance equal to one million ohms. The spelling megohm is also used.
megaohm (megohm) centimeter
a unit of resistivity, used for pure water and for other substances having relatively high resistivity. In the case of water, resistivity is a measure of purity: the higher the purity, the higher the resistivity. The resistivity of a conductor in megaohm centimeters is defined to be its resistance (in megaohms) multiplied by its cross-sectional area (in square centimeters) divided by its length (in centimeters). One megaohm centimeter equals 10 000 ohm meters.
megaparsec (Mpc)
the longest distance unit in common use, the megaparsec is used by astronomers studying the most distant quasars and galaxies. One megaparsec equals one million parsecs, 3.2616 million light years or 30.857 x 1018 kilometers (30.857 zettameters).
megapascal (MPa, MP)
a common metric unit of pressure or stress equal to one million pascals or one newton per square millimeter. One megapascal equals 10 bars or approximately 145.038 pounds per square inch (lbf/in2 or psi) or 20 885.5 pounds (10.443 U.S. tons) per square foot. The symbol MP is used fairly commonly in engineering, but it is not correct: use MPa.
megapixel
a unit used to describe the size or resolution of an image or of a digital camera. One megapixel is one million pixels (picture elements, or "dots"). For example, a rectangular image 1000 pixels by 1000 pixels is comprised of one megapixel.
megapond (Mp)
a metric unit of force equal to 1000 kilograms of force (kgf). The megapond also equals 9806.65 newtons, or 2204.6226 pounds of force in the traditional English system. Although it is considered obsolete, the megapond is still used sometimes by engineers in Europe, especially in Germany.
megaton (Mton or Mt)
a unit of energy used for measuring the energy of an explosion, especially a nuclear explosion. Supposed to be the amount of energy released by the explosion of one million (short) tons of TNT, the megaton is defined to equal 4.18 x 1015 joules (4.18 petajoules), 1.16 billion kilowatt hours, or roughly 4 trillion Btu.
megatonne (Mt)
a metric unit of mass or weight equal to one million metric tons (tonnes), one teragram (Tg), or about 2.2046 billion pounds.
megawatt (MW)
a common metric unit of power. One megawatt is equal to one million watts, about 1341.02 horsepower, or 947.817 Btu per second.
megawatt hour (MW·h)
a metric unit of energy, especially electrical energy. One megawatt hour equals exactly 3.6 gigajoules (GJ), about 3.412 million Btu, or about 2.655 billion foot pounds.
megawatt day (MW·d or MWD)
a unit of energy used in the nuclear power and nuclear weapons industries. One megawatt day equals exactly 24 megawatt hours, 86.4 gigajoules (GJ), about 81.89 million Btu, or about 63.7 billion foot pounds.
megayear (Myr or Ma)
a unit of time equal to one million years.
megohm
a common unit of electric resistance equal to one million ohms. This simplified spelling of megaohm is approved by the Institute of Electrical and Electronics Engineers (IEEE).
meile
A traditional distance unit in German speaking countries, the meile is much longer than the mile units of western Europe. Typically the meile was equal to 4000 klafters (fathoms) or 24 000 fuß (German feet). In Austria this came to 7586 meters (4.714 miles); in northern Germany it was 4.6805 miles or 7532.5 meters. A version of the meile called the geographische meile was defined to equal exactly 4 (Admiralty) nautical miles (24 320 feet, 4.6061 miles, or 7412.7 meters). The geographische meile was designed to equal 1/15 degree [2] or 4/3 league. See also mil [4], the Scandinavian version of this unit.
mel
a unit of perceived musical pitch, originally defined by Stevens, Volkmann, and Newmann in 1937. Our perception of musical pitch is complex. Although tones of higher frequency are perceived as being higher in pitch, tones separated by equal intervals (frequency ratios), such as octaves, will not be perceived as being equally spaced in pitch. A pure tone of frequency 1000 hertz, at a sound level 40 decibels above the faintest sound a listener can hear, is defined to have a pitch of 1000 mels, and tones perceived as being equally spaced in pitch are separated by an equal number of mels. Because perceptions of pitch depend on a number of factors other than frequency, it is not possible to give a straightforward conversion between hertz and mels. For tones above 1000 hertz the perceived pitch in mels is lower than the frequency in hertz; a 10-kilohertz tone is perceived at around 3000 mels. For tones lower than 1000 hertz the perceived pitch is a little higher than the frequency in hertz.
melchior
a huge bottle of champagne, holding about 18 liters.
Mercalli intensity scale
an empirical scale for rating the effects of an earthquake, as opposed to its strength (see magnitude [2] above). Mercalli estimates are stated as Roman numerals (I-XII) to avoid confusion with magnitude estimates on the Richter scale. The scale is named for the Italian geologist Giuseppe Mercalli (1850-1914), who devised the first version in 1902; the modified version used in the U.S. and Canada was developed by Charles F. Richter in 1956.
mercantile pound (lb merc)
a historic English unit of weight, the mercantile pound (libra mercatoria) was the commercial predecessor of the avoirdupois pound [1]. Used from about 1100 to 1300, the mercantile pound contained 15 troy ounces [2] or 7200 grains. This is equivalent to about 1.0286 avoirdupois pounds or 466.55 grams.
-merous
an ending meaning "-parted," added to a number to create an adjective. Thus "8-merous" means "having 8 parts." The suffix, frequently used by botanists, is derived from the Greek meros, "part."
mesh
a traditional unit used to measure the fineness of woven products such as fishing nets, fencing fabric, window screening, etc., equal to the number of strands per inch. For n mesh fabric, the distance between strands is 1/n inch or 25.4/n millimeter.
met
a unit of metabolism. Metabolism, the sum of all the processes going on in the body to sustain life, is measured in units of power expended per unit of body surface area. One met is the metabolism of a seated, resting person, equal to about 58.15 watts per square meter (W/m2) or 13.89 calories per second per square meter (cal/m2·s) regardless of the person's size. Measurements of human metabolism generally fall in the range 0.8-3.0 met, although athletes can achieve 10 met or more.
meter or metre (m)
the metric and SI base unit of distance. Originally, the meter was designed to be one ten-millionth of a quadrant, the distance between the Equator and the North Pole. (The Earth is difficult to measure, and a small error was made in correcting for the flattening caused by the Earth's rotation. As a result, the meter is too short by a bit less than 0.02%. That's not bad for a measurement made in the 1790's.) For a long time, the meter was precisely defined as the length of an actual object, a bar kept at the International Bureau of Weights and Measures in Paris. In recent years, however, the SI base units (with one exception) have been redefined in abstract terms so they can be reproduced to any desired level of accuracy in a well-equipped laboratory. The 17th General Conference on Weights and Measures in 1983 defined the meter as that distance that makes the speed of light in a vacuum equal to exactly 299 792 458 meters per second. The speed of light in a vacuum, c, is one of the fundamental constants of nature. Since c defines the meter now, experiments made to measure the speed of light are now interpreted as measurements of the meter instead. The meter is equal to approximately 1.093 613 3 yards, 3.280 840 feet, or 39.370 079 inches. Its name comes from the Latin metrum and the Greek metron, both meaning "measure." The unit is spelled meter in the U.S. and metre in Britain; there are many other spellings in various languages (see Spelling of Metric Units).
meter-atmosphere
another name for the atmo-meter.
meterlambert or meter-lambert
another name for the nit, an MKS unit of luminance equal to one candela per square meter. The name is coined by analogy with the footlambert.
meter per second (m/s)
the metric and SI unit of speed or velocity. One meter per second is equal to exactly 3.6 kilometers per hour (km/h) or about 2.236 936 miles per hour or 3.280 840 feet per second.
methuselah
a large wine bottle holding about 6 liters, 8 times the volume of a regular bottle.
Metonic cycle
a unit of time equal to 19 years, used in astronomy in predicting the phases of the Moon. By coincidence, 19 years is equal to 6939.602 days and 235 lunar months is equal to 6939.689 days, just 125 minutes longer. As a result, the phases of the Moon repeat almost exactly after 19 years. (Since 19 years can contain either 3 or 4 leap days, the recurrence isn't always exact as to the day of the month.) Many lunar calendars, such as the Chinese and Jewish calendars, share this 19-year cycle. The cycle is named for the ancient Greek astronomer Meton, who first used it for predictions around 433 BC.
metric carat
the current internationally-recognized carat, equal to exactly 200 milligrams.
metric grain
a unit of mass sometimes used by jewelers, equal to 50 milligrams or 1/4 carat. This unit is often used for pearls and is sometimes called the pearl grain.
metric horsepower
a unit of power, defined to be the power required to raise a mass of 75 kilograms at a velocity of 1 meter per second. This is approximately 735.499 watts or 0.986 32 horsepower. The unit is known in French as cheval vapeur, in Spanish as caballo de vapor, and in German as Pferdestärke.
metric hundredweight
an informal unit of mass equal to 50 kilograms or approximately 110.231 pounds, close to the traditional British hundredweight of 112 pounds. This unit is also known by its German name, the zentner, or (in English) the centner.
metric mile
an informal unit of distance used mostly in athletics. The metric mile is equal to 1500 meters or 1.5 kilometers (approximately 0.932 057 statute mile or 4921.26 feet). In U.S. high school competition the term is sometimes used for a race of 1600 meters (0.994 194 miles or 5249.34 feet).
metric pound
an informal name for a mass of 500 grams (0.5 kilogram or 1.1023 pound).
metric quintal
a unit of mass equal to 1 decitonne, 100 kilograms or about 220.462 pounds. See quintal for a more complete description.
metric slug
see TME or hyl.
metric ton (t or MT)
an alternate name for the tonne. In the United States, the Department of Commerce recommends that the tonne be called the metric ton to distinguish it clearly from the traditional American ton. The proper symbol for the unit is simply t.
metric ton unit (mtu)
a unit of mass used in mining to measure the mass of the valuable metal in an ore. Customarily, the metric ton unit is defined to be one metric ton of ore containing 1% metal, but it is the metal, not the ore, that is being measured. Thus the unit is really a unit of mass equal to 10 kilograms (22.0462 pounds).
MeV
the symbol for one million electronvolts. Thanks to Einstein's equation E = mc2 equating mass wth energy, the MeV can be regarded either as a unit of energy equal to 160.217 646 2 femtojoules, or as a unit of mass equal to 1.782 662 x 10-27 gram or 0.001 073 544 atomic mass unit. This is 1.956 951 times the mass of the electron (me).
MFD, mfd
common but incorrect symbols for the microfarad (see below). The correct symbol is µF, or mcF if µ is not available.
mg-at
an obsolete symbol for "milligram atom", an equally obsolete name for the millimole (mmol).
Mgd
an abbreviation for millions of gallons per day (Mgal/d), a unit used in reservoir management to express the rate at which water is withdrawn, or could be withdrawn, for drinking or for some other purpose. 1 Mgd equals approximately 3.785 43 megaliters per day, or 3785.43 cubic meters per day, or 133 681 cubic feet per day.
mg/dl
symbol for milligram per deciliter, a unit used in U.S. medicine to measure the concentration of cholesterol and other substances in the blood. 1 mg/dl equals 0.01 grams per liter (g/L). Internationally, the SI unit for data of this type is millimoles per liter (mmol/L); see the table of SI Units for Clinical Data for conversions of many common measurements.
mg-eq
an obsolete symbol for "milligram equivalent", an equally obsolete name for the milliequivalent (mEq).
mg/kg
symbol for milligram per kilogram, a unit used in medicine to measure dosage rates. 1 mg/kg is equivalent to 10-6 g/g or 1 part per million based on the patient's body weight.
mgon
symbol for the milligon (milligrad), a unit of angle measure equal to 0.001 gon, 10-5 right angle, 0.0009°, 3.24 seconds of arc, or 15.708 microradians (µrad). Surveying equipment is often marked in "mgons."
mho
an older name for the siemens, which is defined to be the reciprocal of the ohm. In case you didn't notice already, "mho" is "ohm" spelt backwards.
mic
an informal name for the microgram, pronounced "mike."
mickey
a unit used in computer science in programming mice and similar input devices. One mickey is the length of the smallest detectable movement of the mouse. This depends on the equipment. Typical values are in the range 1/200 to 1/300 inch or roughly 0.1 millimeter. Obviously, the name comes from the Disney cartoon character Mickey Mouse.
micro- (µ- or mc- or u-)
a metric prefix meaning 10-6 (one millionth). The prefix comes from the Greek prefix mikro-, meaning small. In print the prefix is sometimes abbreviated mc- or u- when the Greek letter mu (µ) is not available.
microampere (µA)
a unit of electric current equal to 10-6 ampere.
microarcsecond (µas)
a unit of angle measurement sometimes used in astronomy. The microarcsecond equals 10-6 arcsecond or about 4.8481 picoradian.
microbar (µbar)
a CGS unit of pressure equal to 0.001 millibar, 0.1 pascal, 1 dyne per square centimeter (1 barye), or about 0.002 089 pounds per square foot. The microbar is used commonly in acoustics and sound engineering.
microcurie (µCi)
a common unit of radioactivity. The microcurie equals 10-6 curie or 37 kilobecquerels; this corresponds to a radioactivity of 37 000 atomic disintegrations per second.
microdegree (µdeg) [1]
a unit of angle measure equal to a millionth of a degree or exactly 36 milliarcseconds.
microeinstein (µE)
a unit of light energy concentration used in measuring the flux or density of light or any form of electromagnetic radiation. The microeinstein is equal to 10-6 einstein or one micromole of photons. The density of photosynthetically active radiation, for example, is reported in microeinsteins per second per square meter (µE/s·m2).
microequivalent (µEq or µeq)
a unit of relative amount of substance equal to 10-6 equivalent weight. This unit is used, for example, in stating the concentrations of ions in drinking water.
microfarad (µF)
a common unit of electric capacitance equal to 10-6 farad.
microflick (µf)
a unit of spectral radiance used in optical and communications engineering, equal to 10-6 flick, or 1 microwatt per steradian per square centimeter of surface per micrometer of span in wavelength. This is mathematically equivalent to 10 milliwatts per steradian per cubic meter.
microgram (µg or mcg)
a metric unit of mass equal to 0.001 milligram (mg) or one millionth of a gram. Ingredients of drugs and vitamins are often stated in micrograms.
microgray (µGy)
a unit of radiation dose equal to a millionth of a gray or 0.1 millirad. Small doses of this size are often provided by natural sources in the environment.
microinch (µin)
a traditional unit of distance equal to 10-6 inch, 0.001 mil, or 25.4 nanometers (nm). The microinch is used rather widely to state the roughness of optical surfaces, precise tolerances in machining, and for other industrial purposes.
microliter (µl, µL, mcl, or mcL)
a metric unit of volume equal to 0.001 milliliter or 1 cubic millimeter (mm3). Microliters are used in chemistry and medicine to measure very small quantities of liquid. This unit has also been called the lambda.
micrometer (µm)
a common metric unit of distance equal to 0.001 millimeter or about 0.039 370 mil. The name micron is also used for this unit.
micromicro- (µµ-)
an obsolete metric prefix denoting 10-12. The prefix has been replaced by pico- (p-).
micromicrofarad (µµF or mmfd)
an older name for the picofarad (10-12 farad). Though it is obsolete now, this name is still seen marked on many capacitors.
micromicron (µµ)
a former name for a millionth of a micron, that is, 10-12 meter. The name bicron was also used for this unit, which is now called the picometer (pm).
micromole (µmol)
a unit of amount of substance equal to a millionth of a mole (see below). This unit is used very commonly in biochemistry, since a mole of a large organic molecule can be quite a large amount.
micron (µ) [1]
a metric unit of distance equal to one millionth of a meter. "Micron" is simply a shorter name for the micrometer. In 1968 the CGPM decided to drop the micron as an approved unit and recommend that micrometers be used instead. Microns, however, are still in common use.
micron (µ) [2]
an informal unit of pressure widely used in vacuum technology. In this use, a micron is a micron of mercury, that is, 0.001 mm Hg or approximately 1.333 microbars (µbar or µb) or 133.3 millipascals (mPa). For all practical purposes, 1 micron is identical to 1 millitorr (mTorr).
micronewton (µN)
a unit of force equal to a millionth of a newton or 0.1 dyne. The unit is often used in astronautical engineering to describe the tiny forces applied to spacecraft to adjust their attitudes in space.
micropascal (µPa)
an SI unit of pressure equal to 10-6 pascal or 1 micronewton per square meter. This very small unit is used to measure the pressure of sound waves.
micropoise (µP, µPo, or µPs)
a unit of dynamic viscosity used primarily for describing the viscosities of gases. One micropoise equals 10-6 poise or 10-7 pascal second (Pa·s).
microrad (µrad)
a unit of radiation dose equal to a millionth of a rad or 10 nanograys.
microradian (µrad)
a unit of angle measure equal to 10-6 radian. The microradian equals about 0.208 533 milliarcsecond (mas).
microrem (µrem)
a unit of effective radiation dose equal to a millionth of a rem or 10 nanosieverts. Doses in this range are much smaller than those provided by natural sources of radioactivity in the environment.
microsecond (µs or µsec)
a unit of time equal to a millionth of a second.
microsievert (µSv)
a unit of radiation dose equal to 10-6 sievert or 0.1 millirem. The radiation doses resulting from exposure to natural sources such as radon gas in the atmosphere are often measured in this unit.
microstrain (µstrain)
a common engineering unit measuring strain. An object under strain is typically deformed (extended or compressed), and the strain is measured by the amount of this deformation relative to the same object in an undeformed state. One microstrain is the strain producing a deformation of one part per million (10-6).
microtesla (µT or mcT)
a common unit of magnetic field intensity equal to 10-6 tesla. The unit is widely used to measure the strength of electromagnetic fields generated by powerlines or electronic equipment. By comparison, the strength of the Earth's own magnetic field at the surface is about 50 microteslas. One microtesla equals 0.01 gauss.
microvolt (µV or mcV)
a unit of electric potential equal to 10-6 volt. This unit is used in cardiology and other medical fields to measure the small potentials within the nervous system.
middy
an informal unit of volume for beer used in many Australian pubs. A middy is generally 285 milliliters (or 10 British fluid ounces), larger than a pony but smaller than a schooner.
miglio
the traditional Italian mile. The miglio equals 1628 yards, which is 0.925 English mile or about 1488.6 meters. This is 32 yards (29.3 meters) shorter than the classical Roman mile.
miil or mijl
alternate spellings for the Scandinavian mil [4] (see below).
mil [1]
a unit of distance equal to 0.001 inch: a "milli-inch," in other words. Mils are used, primarily in the U.S., to express small distances and tolerances in engineering work. One mil is exactly 25.4 microns, just as one inch is exactly 25.4 millimeters. This unit is also called the thou. With the increasing use of metric units in the U.S., many machinists now avoid the use of "mil" because that term is also a handy slang for the millimeter.
mil [2]
a unit of angle measure, used in the military for artillery settings. At one time the U. S. Army used a mil equal to 1/1000 of a right angle, 0.1 grad, 0.09°, or 5.4 arcminutes (often written 5.4 moa; see "moa" below). Later this was changed to 1/1600 right angle, or 0.05625° (3.375 moa). In target shooting, the mil is often understood to mean 0.001 radian or 1 milliradian, which is about 0.0573° or 3.43775 moa. In Britain, the term angular mil generally refers to the milliradian. 1 milliradian corresponds to a target size of 10 millimeters at a range of 10 meters, or 3.6 inches at 100 yards.
mil [3]
a common slang name for the milliliter (mL) or the millimeter (mm).
mil [4]
in Scandinavia, the mil, pronounced like "meal" in English, is a traditional distance unit considerably longer than Roman or English miles. In Denmark, the traditional mil was 24 000 Danish feet, which is 4.6805 miles or 7.5325 kilometers (this is the same as the north German meile; see above). The Danish mil has sometimes been interpreted as exactly 7.5 kilometers (4.6603 miles). In Sweden, the traditional mil was 36 000 Swedish feet, which is 6.641 miles or 10.687 kilometers. In Sweden and Norway the mil is now interpreted as a metric unit equal to exactly 10 kilometers (6.2137 miles). See also sjømil.
mil [5]
an alternate spelling of the mill [1] (see below).
mile (mi) [1]
a traditional unit of distance. The word comes from the Latin word for 1000, mille, because originally a mile was the distance a Roman legion could march in 1000 paces (or 2000 steps, a pace being the distance between successive falls of the same foot). There is some uncertainty about the length of the Roman mile. Based on the Roman foot of 29.6 centimeters and assuming a standard pace of 5 Roman feet, the Roman mile would have been 1480 meters (4856 feet); however, the measured distance between surviving milestones of Roman roads is often closer to 1520 meters or 5000 feet. In any case, miles of similar lengths were used throughout Western Europe. In medieval England, several mile units were used, including a mile of 5000 feet (1524 meters), the modern mile defined as 8 furlongs (1609 meters), and a longer mile similar to the French mille (1949 meters). None of these units corresponded with the Scottish mile (1814 meters) or the Irish mile (2048 meters). In 1592, Parliament settled the question in England by defining the statute mile to be 8 furlongs, 80 chains, 320 rods [1], 1760 yards or 5280 feet. Using the international definition of the foot as exactly 30.48 centimeters, the international statute mile is exactly 1609.344 meters. (In technical U.S. usage, the statute mile is defined in terms of the survey foot and equals about 1609.3472 meters; this unit is called the survey mile). In athletics, races of 1500 or 1600 meters are often called metric miles. See also nautical mile.
mile (mi) [2]
an informal name for mile per hour, sometimes seen on U.S. road signs with markings such as "Speed Limit 25 miles."
mile per gallon (mi/gal or mpg) [1]
the unit customarily used in the United States to measure the fuel efficiency of motor vehicles. 1 mile per U.S. gallon [1] equals about 0.4252 kilometers per liter. In most other countries, however, the usual measure of fuel consumption is liters per 100 kilometers; x miles per U.S. gallon is equal to 235.215/x liters per 100 kilometers.
mile per gallon (mi/gal or mpg) [2]
the unit formerly used in Britain, Canada, Australia, and other British Commonwealth nations to measure the fuel efficiency of motor vehicles, analogous to the U.S. unit but based on the Imperial gallon [3]. Although still used sometimes, this unit has been replaced officially by liters per 100 kilometers; x miles per Imperial gallon is equal to 282.481/x liters per 100 kilometers. One mile per Imperial gallon is equal to about 0.8327 mile per U.S. gallon.
mile per hour (mi/h or mph)
a traditional unit of velocity. One mile per hour equals exactly 22/15 feet per second, approximately 1.609 kilometers per hour (km/h), or exactly 0.447 04 meter per second (m/s).
mil-foot
a mil-foot is a section of wire one foot long and one mil in diameter; this would be a unit of volume equal to about 0.0377 cubic inches or 0.6178 cubic centimeters. However, the unit is used primarily in statements of resistivity in ohms per mil-foot or of density in pounds per mil-foot. The unit is also called the circular mil-foot.
milha
the traditional Portuguese mile, one of the "longest miles" of western Europe at 2282.75 yards (1.297 statute miles or 2087.3 meters).
military pace
another name for a step. In the U.S. Army, the military pace is defined to be exactly 30 inches (76.2 centimeters) for ordinary "quick time" marching and 36 inches (91.44 centimeters) for double time marching. The same definitions are generally used by marching bands.
mill [1]
an informal unit of quantity or of proportion, equal to 0.001. When Congress established the U. S. monetary system in 1791, it provided for 10 mills to the cent and 100 cents to the dollar; thus the mill was an amount of money equal to $0.001. Although the mill is unfamiliar now as a monetary unit, it has come to represent a one thousandth part as a proportion. Many towns in the United States set their property tax rates in mills, for example.
mill [2]
slang for one million.
milla
the traditional Spanish mile, equal to 5000 pies (Spanish feet) or 8 estadios. This is about 1392 meters, 4567 feet, or 0.865 statute mile.
mille [1]
the traditional French mile, equal to 1000 toises. This is equal to about 6394.4 feet, 1.211 statute mile, or 1949 meters. In modern France, the mille sometimes means the nautical mile (mille marin), equal to exactly 1852 meters.
mille [2]
in French-speaking Canada, the English statute mile of 5280 feet (1609.344 meters).
mille [3]
the Latin word for 1000, sometimes used in English in very learned or literary contexts.
millenary
a unit of quantity equal to 1000.
millennium
a traditional unit of time equal to 1000 years. The plural is millennia or sometimes millenniums.
milli- (m-)
a metric prefix meaning 0.001 (one thousandth). The prefix was coined from the Latin number mille, one thousand.
milliampere (mA)
a common unit of electric current equal to 0.001 ampere.
milliampere hour (mA·h)
a common unit of electric charge, used (for example) in stating the capacity of batteries for cell phones and other electronic equipment. One milliampere hour is the charge accumulated by a current of 1 milliampere in 1 hour; this is equal to exactly 3.6 coulombs (C).
milliarcsecond (mas)
a unit of angular measure commonly used in astronomy. One milliarcsecond is equal to 0.001 arcsecond, 0.277 77 microdegrees, or 4.848 137 nanoradians.
milliard [1]
a unit of quantity equal to 109, which is what Americans call a billion. See Using Numbers and Units for more on the "billion" controversy.
milliard [2]
a unit of volume used by engineers to describe a large quantity of water. One milliard equals one cubic kilometer, which is 1 billion (109) cubic meters or about 810 767 acre feet.
millibar (mb)
a common metric unit of atmospheric pressure, equal to 0.001 bar, 100 pascals, 1000 dynes/cm2, about 0.0295 inches (0.7501 millimeters) of mercury, or about 0.014 504 lb/in2. A millibar is the same thing as a hectopascal (hPa), and some weather agencies have replaced the millibar with the hectopascal in an effort to conform with the SI. However, many meteorologists resist this change and continue to use millibars. In fact, an appropriate SI unit for atmospheric pressure would be the kilopascal (10 millibars or 0.145 038 lb/in2).
millicandela (mcd)
a unit of light intensity equal to 0.001 candela. The intensity of the light-emitting diodes (LEDs) used in electronics are stated in millicandelas.
millicurie (mCi)
a common unit of radioactivity. One millicurie represents radioactivity at the rate of 37 million atomic disintegrations per second, that is, 37 megabequerels.
millidegree (mdeg) [1]
a unit of angle measure equal to 0.001° or exactly 36 arcseconds.
millidegree (mdeg) [2]
a unit of temperature equal to 0.001°, usually meaning 0.001 °C.
milliequivalent (mEq or meq)
a unit of relative amount of substance commonly used in chemistry. One mEq equals 0.001 equivalent weight.
millier
a former name for the tonne or metric ton. This name, obsolete now, was used in Britain to avoid confusion with the British long ton.
millifarad (mF)
a common unit of electric capacitance equal to 0.001 farad.
milligal (mGal or mgal)
a unit of acceleration used in geology to measure subtle changes in gravitational acceleration. One milligal equals 10 micrometers per second per second, or 10-5 meters per second per second. The unit should really be called the milligalileo.
milligauss (mG)
a unit of magnetic flux density equal to 0.001 gauss, 0.1 microtesla, or 100 nanoteslas. The magnetic fields generated by power lines and electronic equipment are often measured in milligauss.
milligram (mg)
a very common metric unit of mass equal to 0.001 gram or 1000 micrograms (µg or mcg). One milligram equals approximately 0.015432 grain or 35.274 x 10-6 ounce.
milligram per deciliter (mg/dl or mg/dL)
a conventional unit in medicine for measuring concentrations of cholesterol and many other substances in the blood. Internationally, the SI unit for data of this type is millimoles per liter (mmol/L); see the table of SI Units for Clinical Data for conversions of many common measurements.
milligray (mGy)
a common unit of radiation dose equal to 0.001 gray, 0.1 rad, or 1 millijoule of energy per kilogram of matter. Because the gray itself is such a large unit, many practical radiation measurements are made in milligrays. In particular, the exposures cause by X-ray equipment are typically in the milligray range.
millihenry (mH)
a common metric unit of electric inductance equal to 0.001 henry.
millihg
an informal name (pronounced "millig") for the millimeter of mercury (see below).
millihorsepower (mhp)
a unit of power equal to 0.55 foot-pound per second or 0.7457 watt. This unit is commonly used to state the power of small electric motors.
millijoule (mJ)
a common metric unit of work or energy equal to 0.001 joule or 104 ergs.
milli-k
a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. One milli-k is a reactivity of 0.001 or 0.1%; the origin of the name is that k is a common symbol for reactivity. This unit was introduced in the Canadian nuclear power industry. For a discussion of reactivity, see inhour.
millikelvin (mK)
a unit of temperature equal to 0.001 kelvin or 0.001 degree Celsius (°C). This unit is used mostly by scientists investigating substances cooled very close to absolute zero.
millilambert (mLb)
a common metric unit of illumination equal to 0.001 lambert or 10 lux (lx).
millilux (mlx)
a metric unit of illumination equal to 0.001 lux. The natural illumination at night is measured in millilux.
milliliter (ml or mL)
a very common metric unit of volume. One milliliter equals 0.001 liter, exactly one cubic centimeter (cm3 or cc), or approximately 0.061 023 7 cubic inch or 16.231 U.S. minims (see below). The milliliter is used almost entirely for measuring the volumes of liquids, with solids being measured in cubic centimeters. Note: until 1964 the milliliter was equal to 1.000 028 cubic centimeters; see liter for a discussion of this history.
millimass unit (mu or mmu)
a unit of mass equal to 0.001 atomic mass unit, used in physics and chemistry. This unit is also called the millidalton (mDa). The millimass unit is an SI unit, but its proper SI symbol is mu, not the older symbol mmu.
millimeter (mm)
a very common metric unit of distance. One millimeter equals 0.001 meter, 0.1 centimeter, about 0.039 370 inch, or 39.370 mils.
millimeter of mercury (mm Hg)
a unit of pressure equal to the pressure exerted at the Earth's surface by a column of mercury 1 millimeter high. When a traditional mercury barometer is used, the pressure is read directly as the height of the mercury column in millimeters. One millimeter of pressure is equivalent to approximately 0.03937 in Hg, 0.01933 lbf/in2, 1.333 millibars, or 133.3 pascals. In medicine, blood pressure is traditionally recorded in mm Hg. In engineering, the millimeter of mercury is often replaced by the torr, the two units being equal to within 1 part per million. Hg, the chemical symbol for mercury, is taken from the Latin hydrargyrum, "water-silver," describing the silvery liquid metal.
millimeter of water (mmH2O, mm WC, mm CE, mm WS)
a unit of pressure equal to the pressure exerted at the Earth's surface by a column of water 1 millimeter high. This is a small pressure, about 9.8067 pascals, 0.098 067 millibars, 0.03937 inch of water, or 0.204 pounds per square foot. The French symbol is mm CE (colonne d'eau), and the German symbol is mm WS (Wassersäule).
millimeter of water gauge (mm WG)
another common name for the millimeter of water column. The word "gauge" (or "gage") after a pressure reading indicates that the pressure stated is actually the difference between the absolute, or total, pressure and the air pressure at the time of the reading.
millimicro- (mµ-)
an obsolete metric prefix denoting 10-9 or one billionth. This prefix has been replaced by nano- (n-).
millimicron (mµ)
a former metric unit of distance equal to 0.001 micron or 10-9 meter. The millimicron has been replaced by its equivalent, the nanometer (nm).
millimole (mmol)
a very common unit of amount of substance equal to 0.001 mole (see below).
millimole per liter (mmol/l or mmol/L)
the SI unit in medicine for measuring concentrations of cholesterol and many other substances in the blood. A table is provided for conversion of conventional units, such as milligrams per deciliter (mg/dL), to SI units.
milline
a traditional unit of advertising. One milline equals the height of a line of "agate" type (5.5 points, or about 2 mm) times the width of a column times one million copies of the publication.
millinewton (mN)
a metric unit of force equal to 0.001 newton, 100 dynes, or about 0.101 972 gram of force (gf).
millinile
a unit used in British nuclear engineering to describe the "reactivity" of a nuclear reactor. One millinile is a reactivity of 10-5. For a discussion of reactivity, see inhour.
millioctave (mO)
a unit used in music to describe the ratio in frequency between notes. The difference between two frequencies in millioctaves is equal to 1000 times the base-2 logarithm of the ratio between the two frequences. One millioctave equals exactly 1.2 cents [3] or about 0.30103 savart. If two notes differ by 1 millioctave, the ratio between their frequencies is 21/1000 or approximately 1.000 6934.
millioersted (mOe)
a name sometimes used for the milligauss as a unit of magnetic flux density.
milliosmole (mOsm)
a unit of osmotic pressure, equal to 0.001 osmole, commonly used in biology and medicine.
milliparsec (mpc)
a unit of distance in astronomy equal to 0.001 parsec. Used in studying crowded parts of the universe such as globular clusters and galactic centers, the milliparsec is equal to about 206.265 astronomical units, 11.913 light days, or 30.8568 terameters (30.8568 x 109 kilometers or 49.6486 million miles).
millipascal (mPa)
an SI unit of pressure equal to 0.001 pascal or 1 millinewton per square meter. This very small unit is used to measure the pressure of sound waves.
millipascal second (mPa·s)
an SI unit of dynamic viscosity equal to the centipoise (cP). This unit is gradually replacing the centipoise in many contexts.
milliphot (mph)
a unit of illuminance or illumination equal to 0.001 phot or 10 lux.
millipoise (mP, mPs, or mPo)
a metric unit of dynamic viscosity equal to 0.001 poise or 0.1 millipascal second (mPa·s).
millirad (mrad)
a unit of radiation dose equal to 0.001 rad or 10 micrograys.
milliradian (mrad)
a unit of angle measure equal to 0.001 radian. The milliradian equals about 0.057 296°, 3.437 75 arcminutes, or 3" 26.265'. In Britain this unit is often called the angular mil.
millirem (mrem)
a common unit of radiation dose equal to 0.001 rem or 10 microsieverts (µSv). A millirem is roughly the radiation dose you would receive from wearing a luminous dial watch for a year.
millisecond (ms or msec)
a common unit of time equal to 0.001 second.
millisiemens (mS)
a common unit of conductance equal to 0.001 siemens or 1 milliampere of current per volt of potential difference. The millisiemens is often used to measure the salinity of seawater or brackish water, since adding salt to water makes it much more conductive of electricity.
millisievert (mSv)
a unit commonly used to measure radiation dose. One millisievert equals 0.001 sievert or 0.1 rem.
millitesla (mT)
a common unit of magnetic field intensity equal to 0.001 tesla or 10 gauss. Since the tesla is quite a large unit, many practical measurements are made in milliteslas.
millivolt (mV)
a common unit of electric potential equal to 0.001 volt.
milliwatt (mW)
a common unit of power equal to 0.001 watt.
milliwatt hour (mW·h)
a common metric unit of work or energy, representing the energy delivered at a rate of one milliwatt for a period of one hour. This is equivalent to exactly 3.6 joules (J) of energy, or about 0.003 412 Btu, 0.859 846 (small) calories, or about 2.655 foot pounds.
mina
a historic unit of weight, originating in Babylonia and used throughout the eastern Mediterranean. The mina is roughly comparable to the pound, but over the centuries it varied quite a bit. In Babylonian times it was a large unit, roughly 2 pounds, almost as much as a kilogram. The Hebrew mina, frequently mentioned in the Bible, is estimated at 499 grams (1.10 pounds). The Greek mina was equal to 100 drachmai or 431 grams (0.95 pound). In Biblical times the mina was equal to 60 shekels, and there were 60 minas in a talent.
miner's inch
a traditional unit of water flow in the western United States. The unit originally represented streamflow through an opening one inch (25.4 mm) square at a specified distance below the surface of the water; this distance varied from 4 to 6 inches. In Idaho, Kansas, Nebraska, New Mexico, North and South Dakota, Utah, and Washington the miner's inch is legally defined to equal 9 gallons per minute or 1.2 cubic feet per minute (about 34.07 liters per minute). In Arizona, California, Montana, Nevada, and Oregon the definition is 1.5 cubic feet per minute (42.48 L/min). In Colorado, the legal equivalent is 1.5625 cubic feet per minute (44.25 L/min). See also water inch.
-minex
a suffix used to create small numbers. The number n-minex is 10-n, which is 0.000...0001 with a total of n-1 zeros between the decimal marker and the 1. Thus one millionth (0.000001), for example, is 6-minex. See also dex and -plex.
minim (m or min) [1]
a traditional unit of volume used for very small quantities of liquids. In pharmacy, the term drop traditionally meant the same thing as 1 minim. The minim is defined to be 1/60 fluid dram or 1/480 fluid ounce. The U. S. minim is equal to about 0.003 760 cubic inch or 61.610 microliters, while the British minim is equal to about 0.003 612 cubic inch or 59.194 microliters. As you might guess, the word comes from the Latin minimus, small.
minim [2]
a unit of relative time in music equal to 1/2 whole note (a half note) or 1/4 breve.
minipin
an informal unit of volume for beer and other alcoholic beverages, used mostly in Britain. A minipin is 1/2 of a polypin; this is about 17 Imperial pints or 10 liters (roughly 2.64 U.S. gallons).
minute (′) [0]
a historic unit of proportion equal to 1/60. The Romans lacked our flexible terminology for fractions; they followed Babylonian and Greek practice in visualizing quantities as being divided into 60 parts, so they could express fractions consistently in 60ths. A 60th part was called a pars minuta prima ("first small part") of the whole. For smaller fractions, a 60th part was divided into 60 smaller parts, each called a pars minuta secunda ("second small part"). The minuta prima has come down to us as "minute," the minuta secunda is our "second," and the prima leads to the traditional symbol ′ being called a "prime."
minute (min or ′ or m) [1]
a unit of time equal to 60 seconds or to 1/60 hour. The SI specifies min as the symbol for the time unit and ′ as the symbol for the minute of arc (see below).
minute (′ or m or moa) [2] or minute of arc or minute of angle
a unit of angular measure equal to 60 arcseconds or to 1/60 degree. This unit is often called the arcminute to distinguish it from the minute of time. There are 21 600 arcminutes in a circle. The SI defines min as the symbol for the time unit (see above) and recommends ′ as the symbol for the minute of arc. The symbol moa is often used in target shooting. The international standard ISO 31 requires that angles be stated in degrees and decimal fractions of the degree, without use of arcminutes and arcseconds.
minute (′ or m) [3]
a unit of angular measure used in astronomy. Astronomers measure right ascension (see hour [2]) in time units by dividing the equator into 24 hours instead of 360 degrees. This makes 1 minute of right ascension equal to 15 arcminutes.
minute [4]
a unit of time equal to 1/60 day or 24 minutes in the modern sense. This was the original definition of the minute as a unit of time. The modern definition of 1/60 hour did not appear until the invention of mechanical clocks made it practical to measure such small intervals of time.
minute (min or m or ′) [5]
a unit of sidereal time in astronomy; see sidereal day.
minutum
in medieval times, a unit of time equal to 1/10 hour, or 6 minutes in modern terminology. This unit was divided into 4 moments (see below).
minyan
a traditional Hebrew unit of quantity equal to 10, the number of males aged 13 or over required for a Jewish worship service. (In many modern congregations, both males and females can be included in a minyan.)
mips
a unit of computing power equal to one million instructions per second. An "instruction" is a single program command to the computer's central processor. In a particular computer, there is a definite relationship between the rate at which instructions are processed, in mips, and the "clock speed" of the processor, measured in megahertz (MHz). However, this relationship varies considerably between computers, so it is usually not meaningful to compare the mips rates of dissimilar machines. See also megaflops (above).
mired
a name used in colorimetry for the reciprocal megakelvin (MK-1). The word is an acronym for "micro-reciprocal degree"; it is pronounced my-red, in two syllables.
MIU
symbol for one million international units. Dosages of certain drugs, such as various forms of interferon, are commonly stated in this unit.
MJD
see modified Julian day (below).
mKB
symbol for "meters from the Kelly bushing," used in the oil and gas industry to indicate the length of a bored well as measured from the large bushing at the top of the shaft. Since the drilling is usually not exactly vertical, this measurement will be larger than the actual depth of the bottom of the well. The symbol mTVD is often used for true vertical depth.
mkono
a traditional unit of distance in East Africa, standardized under British rule as 1/2 yard (18 inches, or 45.72 centimeters). This unit is an African version of the cubit.
mkp
a common symbol for the meter kilopond, a metric unit of torque equal to 9.806 65 newton meters (N·m) or 7.233 01 pound feet.
Mlb, Mlbs (1)
common symbols for one million pounds. Although "Mlbs" is seen frequently, symbols need not take the terminal -s in the plural and this dictionary takes the position that they should not; "Mlb" is correct. (Also note that the "l" is not capitalized in the symbol for the pound.)
Mlb (2)
a traditional unit of mass for steam, equal to 1000 (not one million) pounds. This is one of many uses of the Roman numeral M to represent a multiple of 1000; all these uses should be replaced by the metric prefix k- (kilo-).
mM
a common symbol in chemistry for millimolar, that is, millimoles per liter. The SI does not allow the use of this symbol.
MM
an abbreviation for one million, seen in a few traditional units such as those listed below. The abbreviation is meant to indicate one thousand thousand, M being the Roman numeral 1000. However, MM actually means 2000, not one million, in Roman numeration. Since the single letter M is now used commonly for one million, the use of MM is confusing and strongly discouraged.
m/m
an abbreviation for "by mass," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. 2% m/m means that the mass of the substance is 2% of the total mass of the solution or mixture.
MMb, MMbo
symbols for one million barrels of oil; see megabarrel above.
MMBF or MMBM
symbols sometimes used in U.S. forestry for one million board feet. One MMBF represents a volume of 83 333 cubic feet or 2360 cubic meters. "BM" stands for "board measure."
MMBtu
a traditional symbol for one million Btu (about 1.055 057 gigajoules (GJ)), a unit used widely in the energy industry. This unit is also called the dekatherm.
MMcf
a symbol for one million cubic feet (28 316.85 m3, or 28.316 85 megaliters). Similarly, MMMcf is used for one billion cubic feet.
MMcfe
a symbol used in the natural gas industry for one million cubic feet of gas equivalent (cfe). This is really an energy unit equal to about 1.091 terajoules (TJ). Similarly, MMMcfe is used for one billion cubic feet of gas equivalent: 1.091 petajoules (PJ).
MMcps
a symbol used for one million centipoises, a unit of dynamic viscosity. This is a jarring addition of an obsolete English prefix to a metric unit, and its use cannot be recommended. 1 MMcps equals 10 000 poises or 10 kilopoises (kP or kps).
mmg
an obsolete symbol for the microgram, mmg stands for "millimilligram," that is, 0.001 milligram. This symbol should never be used. The SI symbol for the microgram is µg, and an acceptable alternate (often used in medicine) is mcg.
mmHg
symbol for the millimeter of mercury (see above), a unit of pressure equal to 133.3 pascals.
MMM
an abbreviation for one billion (109), seen in a few traditional units such as those mentioned above. The abbreviation is meant to indicate one thousand thousand thousand, M being the Roman numeral 1000. However, MMM actually means 3000, not one billion, in Roman numeration.
MMscfd
symbol for one million standard cubic feet per day, the customary unit for measuring the production and flow of natural gas. "Standard" means that the measurement is adjusted to standard temperature (60 °F or 15.6 °C) and pressure (1 atmosphere).
-mo
a "unit" traditionally used in printing to describe the page size of a book or other publication. In traditional printing, large sheets are printed, folded, and then cut to manufacture the book. After the cut is made, the sheet has been divided into a certain number of "leaves." Each leaf, folded at the spine of the book, comprises two pages front and back. When sheets were cut to form 4, 8, or 12 leaves, the resulting pages were described as quarto (4to), octavo (8vo) or duodecimo (12mo), respectively. Later, the suffix -mo from duodecimo was made into a suffix that can be attached to any number to indicate the number of leaves per sheet; thus 16mo indicates 16 leaves per sheet. Link: book sizes, from Bookbinding and the Conservation of Books, by Matt T. Roberts and Don Etherington, posted by Stanford University.
moa
an acronym for "minute of angle," that is, for the arcminute (see minute [2], above). This unit is commonly used in target shooting to express the angular size of targets or the spacing between marks on a reticle (the grid of lines seen in the eyepiece of a rifle). By coincidence, 1 moa is very nearly equal to a target size of 1 inch at 100 yards; in fact, 1 moa = 1.047 20 inches at 100 yards or 10.4720 inches at 1000 yards. In metric units, 1 moa = 2.9089 centimeters at 100 meters.
modified Julian day (MJD)
a count of days used by astronomers, space agencies, and others. Astronomers have long used the Julian day, a count of days beginning at noon Universal Time January 1, 4713 BC, as a means of specifying a date independent of all calendars. One problem with this is that the numbers are large, more than 2.4 million, for current dates. Also, the old astronomical custom of beginning a day at noon is awkward for converting Julian dates to the ordinary calendar. To ease these problems, space engineers introduced the modified Julian date, equal to the Julian date minus 2 400 000.5. The result is a count of days beginning at 0 hours (midnight) Universal Time on 17 November 1858. Thus (for example) 0 hours 1 January 2005 is MJD 53371.0.
module
a unit of volume for raw cotton in the U.S. When cotton is harvested, machinery is used to compact it into bundles called modules for transportation to the gin. A cotton module is 8 ft by 8 ft by 20 ft, or 1280 cubic feet (about 36.25 cubic meters). This unit is essentially the same as the TEU, the volume of a standard 20 ft container.
Mohs hardness scale
a 1-10 scale for estimating the hardness of a mineral, introduced by the German geologist Friedrich Mohs (1773-1839) in 1812. To apply the scale, one attempts to scratch the mineral with standard minerals assigned hardness numbers as follows: diamond 10, corundum 9, topaz 8, quartz 7, orthoclase 6, apatite 5, fluorite 4, calcite 3, gypsum 2, and talc 1. If, for example, the mineral is scratched by quartz but not by orthoclase, then its hardness is between 6 and 7.
moiety
another name for a half, from the French moitié.
molad
Hebrew name for the lunar (synodic) month (see month [1] below). This unit, 29.530 59 days, is crucial in the regulation of the Jewish lunisolar calendar.
molal (m), molar (M)
these notations, traditionally used by chemists to describe the concentration of chemical solutions, often appear to be units of measurement. It's easy to get them confused. The term "molal" describes the concentration of a solution in moles per kilogram of solvent (mol/kg), while "molar" describes a concentration in moles per liter (mol/L). A solution described as 1.0 µM has a concentration of 1.0 µmol/L. These units are not approved by the General Conference on Weights and Measures. Their use is declining, but still substantial.
molar volume
a unit used by chemists and physicists to measure the volumes of gases. The behavior of gases under ordinary conditions (not at very high pressures or very low temperatures) is governed by the Ideal Gas Law. This law says that the volume V of a gas is related to its temperature T and pressure P by the formula PV = nRT, where n is the number of moles of gas present and the gas constant R equals 8.314 joules per mole per kelvin. The molar volume is the volume one mole of gas occupies at standard temperature (273.16 kelvins, or 0 °C) and standard pressure (1 atmosphere, or 101.325 kilopascals). The molar volume is equal to 22.414 liters or 0.7915 cubic foot. (Occasionally the term "molar volume" is used for the volume occupied by a mole of a substance which is not a gas; in such cases the molar volume will be different for each substance.)
mole (mol)
the SI base unit of the amount of a substance (as distinct from its mass or weight). Moles measure the actual number of atoms or molecules in an object. An earlier name is gram molecular weight, because one mole of a chemical compound is the same number of grams as the molecular weight of a molecule of that compound measured in atomic mass units. The official definition, adopted as part of the SI system in 1971, is that one mole of a substance contains just as many elementary entities (atoms, molecules, ions, or other kinds of particles) as there are atoms in 12 grams of carbon-12 (carbon-12 is the most common atomic form of carbon, consisting of atoms having 6 protons and 6 neutrons). The actual number of "elementary entities" in a mole is called Avogadro's number after the Italian chemist and physicist Amedeo Avogadro (1776-1856). Careful measurement determines Avogadro's number to be approximately 602.214 179 x 1021. In the American system of naming big numbers, that's 602 sextillion 214 quintillion 179 quadrillion, give or take about 50 quadrillion.
moment
a medieval unit of time equal to 1/40 hour or 1.5 minutes. This meaning has come down to us only as "a brief interval of time." The moment was divided into 12 ounces [4] of 7.5 seconds each.
momme [1]
a traditional Japanese weight unit corresponding to the Chinese mace (see above). Jewelers continue to use the momme to measure the weight of cultured pearls; for this purpose it equals exactly 3.75 grams (about 0.132 ounce) or 18.75 carats. The unit is commonly pronounced "mommy" in English.
momme [2]
a traditional unit used to measure the "weight" (density per unit area) of silk. The measure is the weight in momme [1] of a standard strip of silk 25 yards long by 1.49 inches wide, an area of 1341 square inches or about 0.8652 square meter. This makes the silk momme equal to about 3.62 grams per square yard or 4.33 grams per square meter.
MON
abbreviation for motor octane number. See octane number.
mondo point
another name for a millimeter, when used to measure shoe and boot sizes. Ski boots, for example, are sized in mondo points.
month (mo or mon) [1]
a unit of time marked by the revolution of the Moon around the Earth. In many traditional societies the appearance of the first tiny crescent moon after the New Moon signaled the start of the month. This start of the month, based on the first appearance of the Moon, is still proclaimed from mosques in Islamic countries. Thus the lunar month is defined as the average interval between two successive moments of New Moon. Astronomers call this period the synodic month. Its length is 29.530 59 days.
month (mo or mon) [2]
a civil unit of time equal to approximately 1/12 year, but varying from 28 to 31 days [3]. The Sun and the Moon are our traditional time keepers, but they are badly out of step with each other. A solar year equals approximately 12.368 lunar months. The large fraction in this number makes it difficult to design a calendar with a whole number of months in each year. There are at least three solutions to the problem:

[i] Use leap months. In the traditional Chinese and Jewish calendars most years have 12 months, but some have a 13th month. In these luni-solar calendars the length of the year varies from 354 to 384 days.

[ii] Define 12 synodic lunar months to be a year and don't worry about the length of the year. This is the solution of the Islamic calendar. Since the Islamic year has only 354 or 355 days, its length does not match the cycle of the seasons.

[iii] Observe the solar year and let the months be 12 arbitrary periods; don't worry about the Moon. This is the solution adopted by Julius Caesar, who established the civil calendar we use today. In this calendar, all months have 30 or 31 days except the second month, February. February has 28 days in ordinary years and 29 in leap years. See also year [2].

moog
a proposed unit in synthetic music, equal to one volt per octave. The unit would honor Robert Moog (1934-2005), the inventor of the Moog synthesizer.
morgan (M)
a unit of genetic separation used in genetics and biotechnology. If two locations on a chromosome have probability p of being separated during recombination in a single generation, then the distance between those locations is p morgan. In practice, measurements are made in centimorgans, each centimorgan representing a 1% probability. The unit honors the American geneticist Thomas Hunt Morgan (1866-1945), who received the Nobel Prize for Medicine in 1933 for his pioneering work in studying the genetics of the fruit fly Drosophila.
morgen
a traditional unit of land area in Northern Europe. "Morgen" means "morning," and most likely the unit arose as the area a yoke of oxen could plow in one morning. The Dutch morgen, also used in Dutch colonies including old New York, equals about 2.10 acres or 0.850 hectare. In South Africa, this unit was defined to equal 10 246 square yards, which is 2.1169 acres or 0.8567 hectare. In Scandinavia and northern Germany, the morgen is a smaller unit equal to about 0.63 acre or 0.25 hectare (2500 square meters). The Prussian morgen, standardized at 2553.22 square meters, was in common use during the nineteenth century. In Austria and southern Germany, the morgen was often the same as a joch, typically defined to be 0.5755 hectare or about 1.422 acres.
mou
see mu (below).
mouse unit (MU or U)
an unofficial unit of toxicity used in pharmacology. A mouse unit is the dose of a toxin that kills 50% of mice (that is, it is the LD50 dose for mice). Typically the mice are assumed to have a mass of 20 grams, the toxin is administered by intraperitoneal injection, and mortality is measured over a standard period that may vary according to the toxin. The size of the mouse unit (in milliliters or international units) depends on the specific toxin.
MP, MPa
symbols for the megapascal, a unit of pressure or stress (see above). MPa is the correct symbol; MP should not be used.
mppcf
abbreviation for million particles per cubic foot, a unit used to measure concentration of dust particles, mostly in industrial settings. 1 mppcf is equivalent to about 35.315 million particles per cubic meter. The symbol mp/f3 is sometimes used for this unit.
mq
Italian abbreviation for the square meter (metro quadrato). Similarly, cmq is a square centimeter and kmq is a square kilometer. These are non-standard symbols; the correct symbol for the square meter is m2.
msl or MSL
abbreviation for "above mean sea level," often seen in measurements of altitude, as in m (msl) for meters above mean sea level or ft msl for feet above mean sea level. "Mean sea level" is defined to be the average height of the sea, for all stages of the tide, as measured over a 19-year Metonic cycle (see above).
msnm
Spanish symbol for meters above sea level (metros sobre el nivel del mar), a unit of altitude.
Mstb
a symbol commonly used in the oil industry for 1000 stock tank barrels.
msw
symbol for "meters of seawater," a conventional unit of pressure. The pressure exerted by seawater varies slightly with temperature and salinity, but for practical purposes the convention is that each meter imposes a pressure of 0.1 bar or 10 kilopascals (about 0.102 kilograms of force per square centimeter or 1.45 pounds per square inch). Sometimes the convention is that each meter is equivalent to 0.1 atmosphere (0.1013 bar), which is practically the same thing. In English units, 1 msw = 3.28 feet of seawater (fsw). Underwater pressure gauges are frequently calibrated in this unit.
Msym/s
a unit of radio transmission rate equal to 1 million symbols per second.
MT
a common U.S. abbreviation for the metric ton or tonne (1000 kilograms).
mtu
see metric ton unit, above.
mu or mou
a traditional unit of land area in China. The traditional mu is about 675 square meters or 800 square yards. However, the colonial customs authorities used a larger mu equal to 8273.75 square feet, 919.3 square yards, or 768.65 square meters. In modern China, the mu is often reckoned to be exactly 1/15 hectare, which is 666.667 square meters or 797.327 square yards.
mud
a traditional Dutch unit of volume for grains and other dry commodities. Originally varying from market to market, the unit was declared equal to the hectoliter (about 3.5315 cubic feet or 2.838 U.S. bushels) when the metric system was introduced in the Netherlands. With this definition it is still in use.
mug [1]
an informal contraction of "metric slug". See TME or hyl.
mug [2]
another name for a slinch.
mutchkin
a traditional Scottish unit of liquid volume. The mutchkin is about 15 British fluid ounces, which is about 426 milliliters or almost exactly 0.9 U.S. pint.
mwe
abbreviation for meter of water equivalent, a unit used in nuclear physics to describe the shielding around a reactor, accelerator, or detector. 1 mwe of any material (such as rock, gravel, etc.) is a thickness of that material providing shielding equivalent to one meter of water.
MWe, MWt
symbols used in the electric power industry to describe the size of generating plants. MWe is the symbol for the actual output of a generating station in megawatts of electricity; MWt is used for the heat energy, or thermal output, required to operate the generators. Thermal output is typically about three times the electric output.
Mya or mya
a common abbreviation (in English speaking countries) for "million years ago." The form "Mya" is recommended, since the capital M, taken from the metric prefix mega- (M-), is the appropriate symbol for a million.
myria- (my-)
a metric prefix meaning 10 000. This prefix was part of the original metric system of 1795 and was used throughout the nineteenth and early twentieth centuries. It has been obsolete officially since 1960, when the CGPM adopted the standard list of SI prefixes. The ancient Greek word myrios means countless, without number. This was modified by later Greeks to form a word myrioi meaning ten thousand. The word myriad, generally used today to mean an indefinitely large number, originally meant the number 10 000.
myriagram (myg)
a metric unit of mass equal to 10 000 grams or 10 kilograms (about 22.046 pounds). Although it is considered obsolete now, the myriagram was a useful unit comparable to the English quarter or Spanish arroba.
myriameter (mym)
an obsolete metric unit of distance equal to 10 000 meters or 10 kilometers (about 6.2137 miles).

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Checked and revised November 20, 2001