B [1]
informal abbreviation for "billion," generally meaning the American billion 109. This abbreviation is non-metric: the metric abbreviation for 109 is G, standing for the prefix giga-. The B form has been used in such units as Bcf (billion cubic feet) and BeV (billion electron volts).
B [2]
a symbol for international standard paper sizes, followed by the size number, as in B4. Wikipedia has a description of this scale. A sheet of size B n has a width of 100·2-n/2 centimeters and a length of 100·21/2-n/2 centimeters (rounded to the nearest millimeter).
bag [1]
another name for a sack (3 bushels) used as unit of measure in Britain.
bag [2]
an old English unit of weight, varying with the contents of the bag but generally in the range of 2-4 hundredweight (100-200 kilograms).
bag [3]
a unit of weight for cement. Traditionally a bag of portland cement weighs 94 pounds (42.6 kilograms) in the US. and 87.5 pounds (39.7 kilograms) in Canada. However, cement is now being sold also in metric-sized bags of 50 kilograms (110.2 pounds) by many suppliers.
a traditional weight unit in Thailand, now equal to exactly 15 grams or 1/40 catty (0.5291 ounce). The baht, originally the weight of a silver coin of the same name, is used to measure the weight of precious metals. The unit is pronounced bot.
baker's dozen
an informal unit of quantity, equal to 13. Bakers sometimes toss in an extra item for each dozen bought, making a total of 13. This custom is very old, dating at least from the thirteenth century, when the weights and prices of loaves of bread were strictly regulated by royal proclamations called assizes, and bakers could be jailed if they failed to provide fair weight at the listed prices.
bale (bl) [1]
a bundle of merchandise, usually pressed and bound in some way. The word "bale" has been used in many ways to describe standard packages of various commodities. For example, a bale of paper is traditionally equal to 10 reams. In agriculture, a bale of hay is generally a huge round bundle left in the field until needed; these bales can weigh up to 1500 pounds (700 kilograms). In U.S. garden shops, a bale of straw is typically 3 cubic feet (0.085 cubic meter).
bale (bl) [2]
a commercial unit of weight for shipments of cotton. In the United States, one bale of cotton, formerly equal to 500 pounds (226.80 kg), is now equal to 480 pounds (217.72 kg). The British used the Egyptian bale, formerly equal to 750 pounds (340.19 kg) but now equal to 720 pounds (326.59 kg).
a unit measuring the degree of ice coverage of polar seas. One ball equals 10% coverage. The unit was invented by the Russian naval officer N. N. Zhubov (1895-1960).
a unit of density; see degree Plato.
a large wine bottle holding about 12 liters, 16 times the volume of a regular bottle.
banana equivalent dose (BED)
since bananas are rich in potassium they contain a small amount of the naturally occurring radioisotope potassium 40 as well as trace amounts of carbon 14. One banana equivalent dose is the radiation dose carried by a single banana. Obviously it depends on the size of the banana. One BED is usually considered to be 0.1 microsievert (µSv). This unit is often used in informal discussions of naturally occurring radiation sources.
bank cubic meter (BCM)
a traditional unit of volume in coal mining. A bank cubic meter represents the contents of a cubic meter of rock in place, before it is drilled and blasted.
banker's dozen
a humorous unit of quantity equal to eleven; this is a takeoff on the baker's dozen.
bar [1] (b)
a metric unit of atmospheric pressure, equal to one million dynes per square centimeter, 100 kilopascals, 750.062 torr, 1.019 72 kilograms of force per square centimeter (kgf/cm2), or about 14.503 78 pounds per square inch (lbf/in2). The word comes from the Greek baros, "weighty." We see the same root in our word, barometer, for an instrument measuring atmospheric pressure. One bar is just a bit less than the average pressure of the Earth's atmosphere, which is 1.013 25 bar. In practice, meteorologists generally record atmospheric pressure in millibars (mb). In English-speaking countries, barometric pressure is also expressed as the height, in inches, of a column of mercury supported by the pressure of the atmosphere. In this unit, one bar equals 29.53 inches of mercury (in Hg).
bar [2]
another name for the measure, a musical unit.
bar abs, bar dif
symbols indicating pressures measured in bars. bar abs indicates bars absolute, measuring the total pressure including the pressure of the atmosphere. bar dif indicates bars differential, measuring a difference between two pressures, neither of which is atmospheric pressure.
Barcol hardness (BH)
a measure of hardness, primarily for plastics and soft metals, made with a Barcol Impressor. Measurements on the Barcol scale are typically between 30 and 90. There is no precise conversion between these readings and other hardness scales, but the manufacturer provides a data bulletin with tables for approximate conversion to Rockwell, Brinell, and Vickers hardness numbers.
symbol for bar gauge, a common unit of pressure in engineering. The term "gauge" means that the pressure has been read from a gauge that actually measures the difference between the pressure of the fluid or gas and the pressure of the atmosphere.
an informal unit of volume used in the U.S. energy industry. The barges used on American rivers customarily carry about 25 000 barrels of oil (see barrel [2] below). This is equivalent to 1.05 million gallons, roughly 1400 register tons, or about 3975 cubic meters.
an old English unit of length equal to 1/3 inch or about 8.5 millimeters. The custom of using seeds as units of length or weight is very common in farming societies. In Anglo-Saxon England, where barley was a basic crop, barleycorns played this traditional role. The weight of a barleycorn, later renamed the grain, is the original basis of all English weight systems including the older troy system and the later avoirdupois system. As a length unit, 3 barleycorns were equal to the Saxon ynce (inch). The English foot was actually defined as 12 of these ynces, that is, as 36 barleycorns.
bar liter
a metric unit of energy used to measure the potential energy of gases under pressure. The energy is computed by multiplying the volume of gas in liters by the pressure in bars. One bar liter equals exactly 100 joules or about 73.7562 foot pounds. This unit is not acceptable in the SI.
barn (b)
a slightly humorous unit of area used in nuclear physics. When atoms are bombarded with smaller particles such as electrons, the electrons are scattered as if the nucleus of the atom was a tiny solid object. The barn is used to express the apparent cross-sectional area of this scattering object. One barn is equal to 10-28 square meters. Using this unit, physicists can say that such and such a nucleus is "as big as a barn," or 10 barns, or whatever. The proper SI unit to use for these measurements is the square femtometer (fm2); one barn equals 100 fm2.
barrel (bbl or brl or bl) [1]
a commercial unit of volume used to measure liquids such as beer and wine. The official U. S. definition of the barrel is 31.5 gallons, which is about 4.211 cubic feet or 119.24 liters. This unit is the same as the traditional British wine barrel. In Britain the barrel is now defined to be 36 Imperial gallons, which is substantially larger: about 5.780 cubic feet or 163.66 liters. This unit is slightly smaller than the traditional British beer and ale barrel, which held 5.875 cubic feet or 166.36 liters. There are other official barrels, defined in certain U.S. states; most of them fall in the general range of 30-40 gallons. A barrel of beer in the U.S., for example, is usually 31 U.S. gallons (117.35 liters). The origin of the standard symbol bbl is not clear. The "b" may have been doubled originally to indicate the plural (1 bl, 2 bbl), or possibly it was doubled to eliminate any confusion with bl as a symbol for the bale (see above). Note: Some web sites are claiming that "bbl" originated as a symbol for "blue barrels" delivered by Standard Oil in its early days; this is incorrect because there are citations for the symbol at least as early as the late 1700s, long before Standard Oil was founded.
barrel (bbl or bo) [2]
a commercial unit of volume used to measure petroleum. By international agreement a barrel of petroleum equals 42 U. S. gallons, which is about 158.987 liters. The symbol bo (barrel of oil) is used for this unit in the petroleum industry. The petroleum barrel originated in the Pennsylvania oilfields (the first commercial oilfields) in the late nineteenth century. Apparently, 40-gallon barrels were increased to 42 gallons to provide insurance aganst any spillage or underfilling. By coincidence (it seems), this unit is the same size as the traditional tierce, a wine barrel.
barrel (bbl or brl or bl) [3]
a commercial unit of volume used to measure dry commodities such as apples. The U. S. dry barrel, established by Congress in 1912, is 105 dry quarts, which is about 4.083 cubic feet or 115.63 liters. (This is the only case in the United States customary system where a dry volume is less than the corresponding fluid volume.) For certain commodities, other sizes are traditional in the U.S.; for example, a barrel of sugar was traditionally 5 cubic feet (about 141.58 liters). In Newfoundland, a barrel of herring held 32 Imperial gallons (145.47 liters) but a barrel of sand was only 18 Imperial gallons (81.83 liters).
barrel (bbl) [4]
a commercial unit of weight, varying with the commodity being measured. In the U.S., for example, a barrel of flour traditionally holds 196 pounds (88.90 kg) and a barrel of beef, fish, or pork 200 pounds (90.72 kg). A barrel of cement is traditionally equal to 4 bags, which is 376 pounds (170.55 kg) in the U.S. and 350 pounds (158.76 kg) in Canada. In old England, a barrel of herrings was 32 pounds (14.51 kg) and a barrel of soap was 256 pounds (116.12 kg).
barrel bulk
a commercial unit of volume equal to exactly 5 cubic feet or 0.141 584 cubic meters. There are exactly 8 barrels bulk in a freight ton and 20 in a register ton.
barrer or Barrer
a CGS unit of gas permeability for membranes, contact lenses, and similar thin materials. Permeability is defined to be the gas flow rate multiplied by the thickness of the material, divided by the area and by the pressure difference across the material. To measure this quantity, the barrer is the permeability represented by a flow rate of 10-10 cubic centimeters per second (volume at standard temperature and pressure, 0 °C and 1 atmosphere), times 1 centimeter of thickness, per square centimeter of area and centimeter of mercury difference in pressure. That is, 1 barrer = 10-10 cm2·s-1·cmHg-1, or, in SI units, 7.5005 x 10-18 m2·s-1·Pa-1. The unit, often capitalized, honors the New Zealand chemist Richard M. Barrer (1910-1996), who was a leader in research on the diffusion of gases.
a large French wine barrel holding 225 liters (about 59.44 U.S. gallons).
barye (ba)
the CGS unit of pressure, equal to 1 dyne per square centimeter. Pronounced "bar-ee," the barye derives from the Greek word for weight, barys. It's a very small unit; 1 barye is equal to 0.1 pascal (Pa), 1 microbar (µb), or about 14.5 x 10-6 pound per square inch.

Richard Maling Barrer (1910-1996)
New Zealand chemist
basic module (M)
a unit of length equal to 100 mm (1 decimeter) or about 3.937 inches, used in the construction industry. Buildings are increasingly constructed in modular fashion using components that may come from any country. The international standard ISO 2848 Building Construction - Modular Coordination established this decimeter unit and provides rules and principles for its implementation; the idea is that important building dimensions and modular dimensions will be multiples of the basic module. The symbol M is used in the standard instead of the equivalent metric symbols (for example, 85M rather than 8.5 m).
basis point (bp)
a unit of proportion equal to 0.01% or 10-4. The basis point is used in finance to measure small fluctuations in interest rates and the rates of return on investments. Prior to the introduction of the basis point, these fluctuations were measured clumsily in 64ths of a percent.
basis weight
a unit used in the paper industry to express the weight (really, the thickness) of paper. The basis weight is the weight in pounds of one ream (500 sheets) of a basic size sheet. For details, see pound weight.
a traditional unit of weight used in the Ottoman (Turkish) Empire and in Turkic-speaking regions of Russia. The size of the unit varied considerably over time and in various locations, but by the early nineteenth century it had been standardized in the Empire as 6 okas, which is approximately 7.68 kilograms or 16.93 pounds.
abbreviation for bioequivalent allergy unit, a measure of the potency of the compounds used by doctors in allergy skin tests. See PNU.
baud (Bd)
a unit used in engineering for measuring the rate of data transmission over telegraph or telephone lines. The baud rate is the number of distinct symbols transmitted per second, that is, the number of times per second the signal carrying the communication varies in strength or frequency. In the days of the telegraph, the signal increased at the start of a pulse and decreased at the end, so the baud rate was 2 times the number of dots that could be transmitted per second. In recent years, when the baud has been used to represent digital data transmission, the definition has varied according to the technique used. If the symbols transmitted have only two states (on or off) then the baud rate is the same as the transmission rate in bits per second. This was the case in early modems, but in modern data transmission equipment symbols have a much larger number of possible states. For example, if the signal has 28 = 256 possible states, then a single symbol can carry 8 bits of information and the bit rate will be 8 times the baud rate. Consequently, any use of "baud" as a synonym for "bits per second" is incorrect. The baud is named for the French telegraph engineer J. M. E. Baudot (1845-1903), the inventor of the first teleprinter.
a measure of relative density; see degree Baumé.
a common shot pellet size in the U.S.; the diameter of BB shot is used informally sometimes as a distance unit. BB shot has a diameter of 0.180 inch (4.572 mm). See the table of shot pellet sizes.
abbreviations for "before Christ" and "before common era," respectively. These are standard designations for years before the beginning of the common or Christian era (see CE). Oddly, there is no year designated 0 in the common system for numbering years; the year 1 CE (or 1 AD) was preceded by 1 BCE (or 1 BC). The lack of a year 0 means that the number of years elapsing between the year n BCE and the year m CE (or n BC and m AD) is n + m - 1.
a symbol used in the natural gas industry for 1 billion cubic feet of gas equivalent (cfe). This is really an energy unit equal to about 1.091 petajoules (PJ).

Jean-Maurice-Émile Baudot (1845-1903)
French telegraph engineer
bcm [1]
a symbol for "billion cubic microns per square inch," a unit used in flexographic printing. In this process, a roller engraved with a large number of tiny cells is used to transfer ink to the printing plate. The unit measures the total volume of these cells per unit area of the roller. One billion (109) cubic microns per square inch is the same as one cubic millimeter per square inch (mm3/in2) or one microliter per square inch (µL/in2). The appropriate SI unit for this volume per area measure is the micrometer or micron (µm); 1 bcm is equal to 1.5500 µm.
bcm [2]
abbreviation for "bank cubic meter" (see above).
symbol for barrels of oil per day (see barrel [2] above), a unit used in the energy industry to measure the rate at which oil is pumped from a well.
Beach unit
a unit of energy used to measure the puncture resistance of various materials, especially packaging materials. The Beach unit equals 0.0299 joule or about 0.02205 foot pound. The unit is named for the American engineer R.L. Beach, who devised the testing method in the 1930s.
bead sizes
see aught.
beat [1]
a unit of time equal to 0.001 day or 86.4 seconds. "Metric time," meaning decimalized time, is an idea dating back at least to the French Revolution of the 1790's. In most metric time proposals, the day is divided into 10 metric hours, each metric hour into 100 metric minutes (or beats), and each metric minute into 100 metric seconds (sometimes called blinks). In 1998 the Swatch Corporation repackaged metric time in a very attractive way as Internet time. In their proposal, time is counted in beats from midnight Central European Standard (winter) time (2300 Universal Time of the previous day, or 6:00 pm U.S. Eastern Standard Time of the previous day). The time at n beats is recorded as @n.beat; thus midnight U.S. Eastern Standard Time is @250.beat.
beat [2]
a musical unit representing a single rhythmic stress. In most musical compositions, beats are organized into measures, with each measure containing a set number of beats, one of which carries a primary or accented stress. At the same time, the tempo of the music is expressed by setting a number of beats equal to a whole note. If there are m beats in a measure and n beats in a whole note, then the fraction m/n is called the time signature of the composition. For example, a waltz has a time signature of 3/4 (3 beats per measure and 4 beats per whole note), while a march has a time signature of 4/4.
beat [3]
a unit of relative time in acting, representing a short, silent pause for dramatic effect. In the script, playwrights may specify a pause of one, two, or more beats. The unit does not have a definite length, but the director and the actors usually have an intuitive sense of how long a beat should be at a specific point in the action.
Beaufort scale
an empirical scale, first devised by the British admiral Sir Francis Beaufort (1774-1857), for estimating wind speed by observing the effects of the wind. Using the scale, sailors can judge the wind velocity by observing the wind's effects on the waves. There is a corresponding scale for observers on land. For example, a moderate gale (32-38 miles per hour) is described as "force 7" on the Beaufort scale. The Beaufort scale numbers come rather close to being a unit of measurement, because they are equal to the whole number closest to 0.66 times the wind velocity in miles per hour raised to the exponent 2/3 (Beaufort force = 0.66 × (velocity2/3)).

Sir Francis Beaufort (1774-1857)
Irish hydrographer; Rear Admiral Royal Navy
becquerel (Bq)
the SI derived unit of activity, usually meaning radioactivity. "Radioactivity" is caused when atoms disintegrate, ejecting energetic particles. One becquerel is the radiation caused by one disintegration per second; this is equivalent to about 27.0270 picocuries (pCi). The unit is named for a French physicist, Antoine-Henri Becquerel (1852-1908), the discoverer of radioactivity. See also curie. Note: both the becquerel and the hertz are basically defined as one event per second, yet they measure different things. The hertz is used to measure the rates of events that happen periodically in a fixed and definite cycle. The becquerel is used to measure the rates of events that happen sporadically and unpredictably, not in a definite cycle.
beer gallon, beer and ale gallon
see ale gallon.
bee space
an informal unit of distance used in beekeeping. In a hive, bees seal up an opening smaller than a bee space, and they fill a larger opening with new honeycomb. If an opening is equal to a bee space, the bees leave it open as a passageway. A hive can be disassembled to remove the honey if the individual comb frames are carefully spaced one bee space apart. This discovery, made by the British beekeeper Lorenzo Longstroth in 1852, is crucial to modern beekeeping. The exact size of the bee space varies somewhat with the strain of bees being raised, but it is generally very close to 1/4 inch or 6.5 millimeters.
bel (B)
a logarithmic measure of sound intensity, invented by engineers of the Bell telephone network in 1923 and named in honor of the inventor of the telephone, Alexander Graham Bell (1847-1922). If one sound is 1 bel louder than another, this means the louder sound is 10 times more intense than the fainter one. A difference of 2 bels corresponds to an increase of 10 x 10 or 100 times in intensity. The beginning of the scale, 0 bels, can be defined in various ways (see decibel) originally intended to represent the faintest sound that can be detected by a person who has good hearing. In practice, sound intensity is almost always stated in decibels. One bel is equal to approximately 1.151 293 nepers.

Antoine Henri Becquerel (1852-1908)
French physicist
a traditional unit of time. On ships at sea, a practical measure of time is the watch, a period of 4 hours. The watch is divided into 8 bells, so one bell equals 1/2 hour or 30 minutes. Every 30 minutes the ship's bell sounds the number of bells elapsed since the start of the watch.
benz (Bz)
a proposed metric unit of speed, equal to 1 meter per second or about 2.237 miles per hour. The unit would honor Karl F. Benz (1844-1929), the German engineer generally regarded as the inventor of the automobile.
bethe (B)
a unit of energy used in astrophysics to measure the energy released by supernova explosions. One bethe is 1051 ergs or 1044 joules. For comparison, the Sun radiates about 1.2 × 1041 ergs/year, so one bethe represents about 8.3 billion years of solar output. The unit is named for the German-American physicist Hans Bethe (1906-2005). Bethe introduced the unit, but he called it the "foe," an acronym for "(10 to the) fifty one ergs."
a symbol sometimes used in the U.S. for one billion (109) electronvolts. The correct symbol is GeV.
abbreviation for brake horsepower. The brake horsepower of an engine is the effective power output, sometimes measured as the resistance a brake attached to the output shaft provides to an engine. See horsepower.
a common English prefix meaning 2. In statements of frequency, bi- and semi- have become confused and it isn't always clear what a word like "bimonthly" means. This is how it's supposed to work: in adverbs of frequency, bi- means "every two." Thus a biweekly payroll is paid once every two weeks, a bimonthly magazine is published once every two months, and the U.S. House of Representatives is elected biennially (every two years). For something that happens twice per time unit, use semi-.
a confusing expression of frequency. The word is used both for twice a year and for once every two years, so it should be avoided. Twice a year is semiannually and once every two years is biennially.
bicron (µµ)
an obsolete metric unit of distance, defined as 10-12 meter or 10-6 micron. This distance is now called a picometer (pm). The name is a reference to the double prefix in "micromicron."
BID or b.i.d.
abbreviation for the Latin phrase bis in die, twice a day, a unit of frequency traditionally used by doctors in writing medical prescriptions.

Hans Bethe (1906-2005)
German-American physicist
a unit of time equal to two years. Many U.S. states, including North Carolina, elect legislators every two years and adopt budgets for this two-year period, called a biennium.
a traditional unit of land area in South Asia. The bigha varies in size from region to another; in India it is generally less than an acre (0.4 hectare). In Bengal (both in Bangladesh and in West Bengal, India) the bigha was standardized under British colonial rule at 1600 square yards (0.1338 hectare or 0.3306 acre); this is often interpreted as being 1/3 acre. In central India bighas were standardized at 3025 square yards or 5/8 acre (0.2529 hectare). In Nepal the bigha equals about 0.677 hectare (1.67 acres). The bigha was divided into 20 katthas, and each kattha contained 20 dhurs.
a new word meaning one billion (109) years. The plural billennia is appearing informally in science writing in phrases such as "countless billennia" or "the billennia of evolutionary time."
a single stick of firewood. Traditional English billets had a length of 40 inches (1.016 meters).
a unit of quantity equal to 1015, which is one quadrillion in American terminology or 1000 billion in traditional British terminology. The name is coined to parallel milliard, which has long been a name for 1000 million.
billion (bn or B)
a number equal to 1000 million in the U.S. but one million million in traditional British usage. See Names for Large Numbers for a discussion of the billion problem. The symbol bn is often used for billion, especially in Britain; Americans also use B.
a rarely-seen unit of time equal to 2 months.
a unit of time equal to 2000 years.
an old English unit of quantity for eels, equal to 250.
biot (Bi)
another name for the abampere, a unit of electric current equal to 10 amperes. This unit honors the French mathematician and physicist Jean-Baptiste Biot (1774-1862), one of the founders of the theory of electromagnetism.
informal name for the bit per second (see below).
bissextile year
an old name for a leap year. In the Roman calendar as proclaimed by Julius Caesar in 45 BC, the leap ("intercalary") day in a leap year was inserted not at the end of February but after the day known as ante diem sextum Kalendas Martii (February 24). The leap day was known as ante diem bis sextum Kalendas Martii, hence the bissextile day, and the year was the bissextile year.
bit (bit or b) [1]
the basic unit of the amount of data. Each bit records one of the two possible answers to a single question: "0" or "1," "yes" or "no," "on" or "off." When data is represented as binary (base-2) numbers, each binary digit is a single bit. In fact, the word "bit" was coined by the American statistician and computer scientist John Tukey (b. 1915) in 1946 as an acronym for binary digit. Somewhat more generally, the bit is used as a logarithmic unit of data storage capacity, equal to the base-2 logarithm of the number of possible states of the storage device or location. For example, if a storage location stores one letter, then it has 26 possible states, and its storage capacity is log2 26 = 4.7004 bits. Note: the symbol b commonly used for the bit is easily confused with the symbol B for the byte (one byte is equal to 8 bits). To avoid this confusion the International Organization for Standardization (ISO) in its standard ISO 80000-1 specifies that bit should be its own symbol, so that, for example, the symbol for the kilobit is kbit rather than kb.
bit (b) [2]
a unit of information content, now known also as the shannon. In information and communications theory, if a message has probability p of being received, then its information content is -log2 p shannons. This unit is often called the bit, because if the message is a bit string and all strings are equally likely to be received, then the information content is equal the number of bits.
bit per second (b/s or bps)
a common unit of data transmission rate in computer science. The symbol bps is often pronounced "bips." Modem transmission rates are often stated in kilobits per second (Kbps or kb/s) or megabits per second (Mbps of Mb/s). See also baud (above).
a unit of time equal to 10-5 day or exactly 0.864 second. This unit is also called the metric second; in most metric time proposals, the day is divided into 10 hours, each hour into 100 minutes (called beats in "Internet time": see "beat" above), and each minute into 100 seconds or blinks. An actual eye blink takes less than half as much time as this unit.
block [1]
an informal unit of distance popular in the U.S. A block is the distance between street intersections in the rectangular street grids common in many American cities. The length of a block varies from about 1/20 mile (80 meters) to about to about 1/10 mile (160 meters).
block [2]
a "unit" used in describing the distance between two locations in a city by the number of intersections encountered between those locations; in this usage "12 blocks" means the traveler will cross 12 street intersections before reaching the destination.
block [3]
an informal unit of area equal to one square block [1]. A typical block has an area of roughly 2-5 acres or 1-2 hectares.
another name for an apostilb. Proposed in 1942, this unit honors the French physicist André Blondel (1863-1938) for his pioneering work in photometry.
blood alcohol level (BAL)
a legal measurement of alcohol concentration in the bloodstream, determining whether a person is considered legally impaired or intoxicated. The blood alcohol level is usually stated as a percentage, such as 0.10%. In most U.S. states, this is a measurement of "weight by volume" (w/v). 0.10% w/v is equivalent to 100 milligrams of alcohol per deciliter (100 milliliters) of blood, which is the same as 1 gram per liter (g/L). In a few states, the measurement is the percentage mass concentration (w/w, or milligrams of alcohol per milligram of blood). 0.10% w/w is the same alcohol concentration as 0.1055% w/v or 1.055 g/L, a slightly higher concentration than 0.10% w/v. Internationally, blood alcohol levels are often stated in millimoles per liter (mmol/L). 1 millimole per liter is equal to 4.61 mg/dL or 0.00461% w/v. In the other direction, each 0.01% w/v is equal to about 2.169 mmol/L.
a common symbol for billion, generally the U.S. billion (109).
board foot (bd ft, fbm, or BF)
a unit of volume used for measuring lumber. One board foot is the volume of a one-foot length of a "standard board" twelve inches wide by one inch thick. Thus a board foot equals 144 cubic inches, or 1/12 of a cubic foot, or approximately 2.360 liters. The number of board feet is easily computed from the dimensions of the stack, no matter how wide or thick the boards are. For example, a stack of two-by-fours 4 ft high, 4 ft wide, and 8 ft long contains 4 x 4 x 8 = 128 cubic feet, equivalent to 128 x 12 = 1536 board feet. The symbol fbm is an abbreviation for "foot, board measure."
Board of Trade unit (BOTU)
a British name for the kilowatt hour, a unit of electrical energy.
body inch
an English name for the t'sun [2], the basic distance measurement used in acupuncture.
body mass index (BMI)
a measure of "fatness" used in medicine and health. The BMI is equal to a person's mass ("weight") M (in kilograms) divided by the square of his or her height H (in meters): BMI = M/H2. If measurements are made in traditional English units, the equivalent formula is BMI = 703.07·M/H2, where M is measured in pounds and H in inches. In the U.S., a person with a BMI less than 20 is considered underweight, a person with a BMI of 25 or more is considered overweight, and a person with a BMI of 30 or more is considered obese. (These figures are for adults. Doctors also keep in mind that women's BMI measures are normally lower than those of men.)
abbreviation for "barrel of oil equivalent," a commercial unit of energy. One boe is equal to about 6.119 gigajoules (GJ) or 5.800 million Btu.
bohr or bohr radius (a0)
a unit of distance used in particle physics. The bohr radius represents the mean distance between the proton and the electron in an unexcited hydrogen atom. It equals about 52.918 picometers (pm), or 52.918 x 10-12 meter. The unit is named for the Danish physicist Niels Bohr (1885-1962), who explained the structure of atoms in a famous paper in 1913.
boiler horsepower
a traditional unit measuring the power delivered by a boiler. The boiler horsepower is defined to be the power required to convert 30 pounds (13.61 kilograms) per hour of water at 100 °F (37.78 °C) to saturated steam at a pressure of 70 pounds per square inch gauge (482.6 kilopascals gauge). This power, about 33 471 Btu per hour or 9.8095 kilowatts, is more than 13 times the usual mechanical definition of the horsepower, but it is judged sufficient to run an engine producing one horsepower of mechanical power.
a rarely-used (and probably obsolete) cgs unit of momentum. The bole is equal to 1 gram-centimeter per second (g·cm/s), or 10-5 kilogram-meter per second (kg·m/s) in SI units.
a traditional unit of volume in Scotland equal to 16 Scots pecks or 4 firlots. This is about 145 liters for wheat, peas, or beans, or about 194 liters for oats or barley.
a commercial unit of length or area used to measure finished cloth. Generally speaking, one bolt represents a strip of cloth 100 yards (91.44 meters) long, but the width varies according to the fabric. Cotton bolts are traditionally 42 inches (1.067 meters) wide and wool bolts are usually 60 inches (1.524 meters) wide. Thus a bolt of cotton is 116.667 square yards (97.566 m2) and a bolt of wool is 166.667 square yards (139.355 m2).
bone-dry unit (bdu)
a commercial unit of quantity of product used in the forest products industry to measure bulk products such as wood chips. One bone-dry unit is the quantity of wood chips (or whatever) that would weigh 2400 pounds (1.0886 metric ton) if all the moisture content were removed. The bone-dry ton (bdt) is similar but based on a weight of 2000 pounds (0.9072 metric ton). The bone-dry metric ton (bdmt) is based on a weight of one tonne (2204.623 pounds).
book sizes
see -mo; also see the much more complete information posted by Stanford University.
the bore of a gun is the inside diameter of its barrel. As a result, "bore" is sometimes used as a unit, meaning the same as gauge [1] for shotguns and the same as caliber [1] for other guns.
bottle (btl)
a unit of volume. Like actual bottles, this unit varies according to the nature of the contents. For a long time in the U.S., a bottle of milk was 1 quart (1/4 gallon or 946.36 milliliters), a bottle of whiskey was 1 fifth (1/5 gallon or 757.1 milliliters), and a bottle of champagne was 2/3 quart (1/6 gallon or 630.91 milliliters). In the British Empire, a common bottle size was 2/3 Imperial quart (1/6 gallon or 757.68 milliliters), a unit known as the reputed quart. Today wine is customarily sold in bottles containing 750 milliliters (about 25.3605 U.S. fluid ounces or 26.3963 British Imperial fluid ounces). See also quart [2]. Other countries have traditional units of about this size. For example, the Russian boutylka contains 768.95 milliliters.
bougie (bg or cd)
French word for "candle," also used historically for the candlepower and currently for the candela. French authors should use the SI symbol cd and not "bg" when they use this term to mean the candela.
an old English unit of land area equal to 1/8 hide. This was roughly 15 acres or 6 hectares. The word comes from the Latin bovis, an ox, indicating that the bovate was an area that could be farmed with the help of one ox.
a unit of volume, usually informal but standardized in certain industries. In the U.S., a box of citrus fruit contains 1.6 bushels (56.383 liters).
abbreviation for "years before present," a unit of time used in anthropology, geology, and paleontology. By international convention, the year 1950 CE (or AD) represents the present, so years "BP" are really years before 1950.
BP unit
a unit used in Britain to measure the potency of a vitamin or drug, that is, its expected biological effects. For each substance to which this unit applies, the British Pharmacopoeia Commission has determined the biological effect associated with a dose of 1 BP unit. Other quantities of the substance can then be expressed in terms of this standard unit. In many cases, the BP unit is equal to the international unit (IU). Since the British Pharmacopoeia includes the standards of the European Pharmacopoeia, BP units are equivalent to "Ph Eur" units for those substances covered by both sets of standards.
abbreviation for barrels per day, a unit of production used in the petroleum industry.
abbreviation for beats per minute, the common unit of tempo in music. The same symbol is used in medicine for heart rates and also for respiration rates (breaths per minute). Technically, 1 bpm is equal to 1/60 hertz.
see braza, below.
a traditional Italian unit of distance. The word means "arm", and a braccio is the length of a man's arm, about 27 or 28 inches (68-71 centimeters). In modern times, the braccio has become an informal metric unit of exactly 70 centimeters (about 27.56 inches).
another name for a pair. In football (soccer) a brace is two goals scored in the same game by a single player. The word is also used by hunters, who may speak of a brace of partridges or a brace of shotguns. Derived from the Latin word bracchia for both arms, it literally means "one for each arm."
brake horsepower (bhp)
the effective power output of an engine, sometimes measured as the resistance a brake attached to the output shaft provides to an engine. See horsepower.
a traditional unit of distance in France, comparable to the English fathom. The brasse is equal to 5 pieds (French feet); using the pied de roi as the standard for the pied this is about 1.624 meters or 5.328 English feet, slightly shorter than the Spanish braza (next entry). Note that the French have another fathom-size unit, the toise, equal to 6 pieds (about 1.949 meters or 6.395 English feet). The brasse was commonly used at sea and the toise on land. The name of the unit is related to bras (arm) and brassée (armful), recalling the traditional definition of the fathom as a man's armspan.
a traditional unit of distance in Spain and Latin America. Like the brasse, the braza is comparable to the English fathom. In Spain it equals 2 varas, 8 palmos, or about 1.67 meters (5.48 feet or 65.75 inches). In Latin America the braza tends to be larger. The Argentine braza is 1.73 meters (5.68 feet or 68.16 inches), and under the Texas definition of the vara the braza would be 1.693 meters (5.556 feet or 66.67 inches). The Portuguese braça is similar, but it equals 10 palmos or about 2.20 meters (7.22 feet or 86.6 inches).
another name for a span (9 inches or 22.86 centimeters). This unit is traditionally used to measure the dimensions of flags.
breakfast cup
a unit of liquid volume, used in food recipes in Britain. The breakfast cup is similar to the cup used by American cooks, except that it is based on British Imperial units. Thus it equals 1/2 Imperial pint. This is equivalent to 10 Imperial fluid ounces, 17.339 cubic inches, 1.20 U.S. cup, or about 284 milliliters. This unit is also called a tumblerful.
the standard unit of relative time in music, equal to the time length of 2 whole notes. Although this is the longest interval in musical notation, the word comes from the Latin brevis, brief. The breve is equal to 2 semibreves, 4 minims, 8 crotchets, or 16 quavers.
brewster (B)
a unit used in optics to measure stress-induced birefringence. A material exhibits birefringence if it has different indices of refraction along different axes. Many crystals exhibit birefringence. In addition, certain substances, including many plastics, exhibit birefringence if they are subjected to compression or stress in one direction. The brewster, which measures this effect, has dimensions reciprocal to those of stress. One brewster is defined to equal 10-12 square meters per newton (m2/N) or 10-13 square centimeters per dyne (cm2/dyn). The unit is named for the British physicist David Brewster (1781-1868), who discovered stress-induced birefringence in 1816.
bright, brightness
a unit describing surface brightness, or reflectivity, especially of paper. The fraction of light reflected by a surface is called its albedo, and its brightness is the albedo expressed as a percentage. Thus "88 bright" paper reflects 0.88, or 88%, of the light falling on the paper.
a unit used to express the "brilliance" or subjective brightness of a source of light. The scale used is logarithmic: an increase of 1 bril means doubling the luminance (and thus the actual amount of light energy) emitted by the source. A luminance of 1 lambert is defined to have a brilliance of 100 brils. Mathematically, the brilliance in brils equals (log L)/log 2 + 100, where L is the luminance in lamberts.
Brinell hardness (HB or BHN)
a measure of the hardness of a metal introduced by J. A. Brinell in 1900. In the Brinell test (generally used for metals of uniform hardness), a hard object such as a steel ball is pressed into the material being tested. The ball is of a specified diameter, usually 1 centimeter. The Brinell hardness is the amount of force applied to the ball divided by the area of the indentation the ball makes in the material. The result is measured in kilograms of force per square millimeter but should be stated as an empirical reading, without units. For readings up to about HB 500, Brinell hardness is about 0.96 times the Vickers hardness.
British horsepower (Bhp)
the traditional horsepower, equal to about 745.7 watts, as opposed to the metric horsepower, which is equal to about 735.5 watts. Note that the symbol bhp, with the lower case "b," is used for brake horsepower.
British shipping ton
see shipping ton.
British thermal unit (Btu or BTU or BThU)
a unit of heat energy defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In America the British thermal unit is sometimes called the heat unit. In defining the Btu, it is necessary to specify the temperature of the water; thus there have been several definitions over the years. However, one Btu is equal to about 251.996 (small) calories, or 0.251 996 of the (kilo-)calories counted by dieters. Using the current definition of the calorie (the IT calorie), one Btu equals approximately 778.169 foot pounds, 1.055 056 kilojoules or 0.293 071 watt hour. The symbol BThU has also been used, especially in Britain.
a unit of proportion, equal to percent, used in measuring the concentration of sugar and other soluble solids in unfermented grape juice, fruit syrups, maple syrup, and similar solutions. Thus one brix equals 1%. The unit is named for the Austrian scientist Adolf Brix (d. 1870), who invented a hydrometer that reads directly the percentage of sugar in the juice, provided the reading is taken at a specified temperature. Also called degree Brix (°Bx). In winemaking, the alcohol concentration in the finished wine is estimated to be 0.55 times the sugar concentration in brix of the unfermented juice. In the U.S. citrus fruit industry, juice concentrate is generally required to contain at least 42 brix of soluble solids (sugars and acids).
a non-standard symbol used in the U.S. heating and airconditioning industry for British thermal units per hour, a unit of power. 1 Btuh equals 0.293 071 watt. The proper symbol is Btu/h.
Bubnoff unit (B)
a unit used in geology to measure erosion rates. One Bubnoff unit is equal to the removal of one micrometer per year (µm/a), or one millimeter per thousand years, or one meter per million years (m/Ma). First proposed in 1968, the unit is named for Serge von Bubnoff (1888-1957), a Russian-born geologist who taught in Germany.
a unit of volume, generally informal. In the U.S., many commodities (both wet and dry) are sold in plastic buckets holding 5 U.S. liquid gallons (about 18.927 liters). In Britain, a bucket is often understood to be 4 gallons, or 18.182 liters based on the British Imperial gallon.
a traditional Dutch unit of land area. Since the adoption of the metric system in the Netherlands in 1809 the bunder has been considered equal to the hectare (2.471 acres). Historically, the unit varied with locality, generally in the range 0.85-1.3 hectare. The unit was also used in Belgium, being called the bonnier in French-speaking provinces.
bundle (bdl) [1]
a unit of quantity for paper, equal to 2 reams or 40 quires. This would be 960 sheets using the old definition of 24 sheets per quire, or 1000 sheets using the newer quire of 25 sheets.
bundle (bdl) [2]
a traditional unit of length for yarn, equal to 20 hanks. For cotton yarn, a bundle contains 16 800 yards (about 15.362 kilometers). For wool, a bundle contains 11 200 yards (10.241 kilometers).
bundle (bdl) [3]
in the construction trades, a bundle is a package of shingles. Shingles are usually packed so that exactly 3, 4, or 5 bundles are needed to cover a square (100 square feet or 9.29 square meters). Asphalt shingles are often sold in bundles of 27, with 3 bundles per square, but the heavier cedar shingles generally require 5 bundles per square.
bushel (bu) [1]
a traditional unit of volume used for measuring dry commodities such as grains and fruits. In the United States, the customary bushel is based on an old British unit known as the Winchester bushel. This unit dates to the early fourteenth century, at least: King Edward I defined the bushel to be 8 gallons in 1303. The form used in the U.S. was legalized by Parliament in 1696. One U.S. or Winchester bushel equals 4 pecks or 32 (dry) quarts; this is a volume of 2150.42 cubic inches or about 1.2445 cubic feet, and represents the volume of a cylindrical container 18.5 inches (47.0 cm) in diameter and 8 inches (20.3 cm) deep. The U.S. bushel holds about 35.239 07 liters. Traditionally, there is also a heaped bushel, which is 27.8% (sometimes 25%) larger than a regular bushel. The regular bushel is also called struck measure to indicate that the bushels have been struck, or leveled, rather then heaped. The origin of the word "bushel" is unclear; some scholars believe it derives from an ancient Celtic unit, but most believe it is of medieval French origin, probably a slang name for a wooden crate (the French word for wood is bois).
bushel (bu) [2]
a unit of volume in the British Imperial system (see gallon [3]) equal to 8 Imperial gallons, or 2219.36 cubic inches (1.2844 cubic feet). The Imperial bushel holds about 36.369 liters.
bushel (bu) [3]
a commercial unit of weight for grains and other bulk commodities. Agricultural commodities such as wheat are traditionally sold by the bushel, but because commodities tend to settle and compact in shipping, disputes over the volume delivered arise easily. To avoid these disputes, traders in a market or a country generally agree on a standard weight for one bushel of the commodity. Often this standard weight is set by law. Although the bill of lading still shows "bushels," it is really the weight rather than the volume that is sold and guaranteed. For example, in the United States a bushel of wheat equals 60 pounds (27.216 kg), a bushel of barley 48 pounds (21.772 kg), a bushel of oats 32 pounds (14.515 kg), and a bushel of rye 56 pounds (25.401 kg). Circular 780 of the Georgia Farm Bureau has a table of standard bushel weights for a range of commodities.
bushel per acre (bu/ac)
a common unit of crop yield in U.S. agriculture. For a crop having a bushel weight of b pounds, one bushel per acre is about 1.1208·b kilograms per hectare in metric units.
business day
a unit of time equal to one day [3] during which a business is open. Phrases such as "8-10 business days" are common; they refer to a period of time containing that number of business days plus however many days the business is not open during the period; thus "8 business days" may mean 10-12 actual days, or even longer if holidays intervene.
a unit of volume for beer in South Australia. A butcher of beer is a glass holding 200 milliliters (about 7 Imperial fluid ounces). This is called a glass [3] or a seven in other parts of Australia.
a traditional unit of volume used for wines and other alcoholic beverages. A butt is generally defined to be two hogsheads, but the size of hogsheads varies according to the contents. In the United States a hogshead is typically 63 gallons and a butt is 126 gallons: about 16.844 cubic feet or 476.96 liters. In Britain, a butt of beer is 108 Imperial gallons: about 17.339 cubic feet or 490.98 liters. The word comes from the Roman buttis, a large cask for wine.
button measure
a unit of distance equal to 1/40 inch (0.635 millimeter), used, as you might guess, for measuring the thickness of buttons. This unit is also called the line, a bad idea since the line is generally equal to 1/12 inch.
Bya or bya
a common abbreviation (in English speaking countries) for "billion years ago."
byte (B)
a unit of information used in computer engineering. Technically the byte is a unit of addressable memory, and its size can vary depending on the machine or the computing language. However, in most contexts the byte is equal to 8 bits (or 1 octet) and this size was codified by the international standard ISO 2382 in 1993. This means that a byte has 28 = 256 possible states. The unit was named by IBM engineer Werner Buchholz in 1956, and the 8-bit size was popularized starting in 1964 by IBM's System 360, a top-selling mainframe computer. The spelling "byte" is used instead of "bite" in order to avoid confusion with the bit.


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