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


a mathematical number equal to the square root of -1. Although often called the imaginary unit, i is quite real in many applications. For example, in vector geometry it is used to represent a counterclockwise rotation by 90°. The Swiss mathematician Leonhard Euler (1707-1783) introduced the symbol i for the imaginary unit in 1777.
an abbreviation for International Annealed Copper Standard, a measure of conductivity used to compare electrical conductors to a traditional copper-wire standard. Conductivity is expressed as a percentage of the standard. 100% IACS represents a conductivity of 58 megasiemens per meter (MS/m); this is equivalent to a resistivity of 1/58 ohm per meter for a wire one square millimeter in cross section.
an abbreviation for international bitterness (or bittering) unit, a unit used in brewing beer and ale. One IBU is equal to 1 part per million of isohumulone, an acid (derived from hops) that provides the bitterness in the brew. Measurements from 0 to about 70 IBU are possible, but most beers measure between 10 and 30 IBU.
abbreviation for inlet cubic feet per minute, a unit traditionally used to measure the capacity of air compressors.
symbol for international einheit, the German name for the international unit (IU).
abbreviation for Imperial gallons per minute.
a traditional Swiss unit of liquid volume. The immi has been brought into the metric system and now equals exactly 1.5 liters (about 3.17 U.S. liquid pints).
a large wine bottle holding about 6 liters, 8 times the volume of a regular bottle. Also called a methuselah.
Imperial gallon (gal or IG)
a traditional unit of volume equal to about 1.201 U.S. liquid gallon or 4.546 liters. See gallon [3]. In Canada, the term "Imperial" is used frequently to distinguish the British Imperial units from the corresponding U.S. units.
Imperial units
the units of the British Imperial system, adopted by Parliament in 1824. The basic units of the system are the foot, the avoirdupois pound, and the Imperial pint. The Weights and Measures Acts of 1963 and 1985 have redefined the Imperial units in term of the SI units. The Imperial units remain in varying degrees of use in Britain, Canada, Australia, and other areas of British heritage despite the introduction of metric units there. Americans should note that the Imperial foot and pound are essentially the same as the units used in the U.S., but the Imperial pint is significantly larger than the U.S. liquid and dry pints.
inch (in or ") [1]
a traditional unit of distance equal to 1/12 foot or exactly 2.54 centimeters. The Old English word ynce is derived from the Latin uncia, meaning a 1/12 part; thus "inch" and "ounce" actually have the same root. The inch was originally defined in England in two ways: as the length of three barleycorns laid end to end, or as the width of a man's thumb at the base of the nail. The barleycorn definition is peculiarly English, but the thumb-width definition is generic. In fact, in many European languages the word for inch also means thumb: examples include the Dutch duim, Swedish tum, French pouce, and Spanish pulgada. In the history of English units the inch seems to come before the foot: after the Norman conquest of 1066 the foot was defined to equal 12 inches, rather than the inch being defined as 1/12 foot.
inch (in or ") [2]
one of several traditional units of pressure. Air pressure is measured traditionally in inches of mercury (next entry) and water pressure in inches of water column (following entry).
inch of mercury (in Hg)
a traditional unit of atmospheric pressure. In the United States, atmospheric pressure is customarily expressed as the height of a column of mercury exerting the same pressure as the atmosphere. When a traditional mercury barometer is used, this height is read directly as the height of the mercury column. These readings must be corrected for temperature since mercury, like most liquids, tends to expand as it warms. The conventional equivalent of one inch of mercury is 0.491 153 pounds per square inch or 3.386 38 kilopascals (33.8638 millibars). In the symbol for the unit, Hg is the chemical symbol for mercury; it comes from the Latin word hydrargyrum ("water-silver") for the liquid metal.
inch of water column (in WC)
a traditional unit of pressure, used in plumbing to describe both water and gas pressures. The conventional equivalent of one inch of water is 249.0889 pascals, which is 2.490 889 millibars, about 0.036 127 pounds per square inch (psi) or about 0.073 556 inches (1.868 32 millimeters) of mercury.
inch of water gauge (in wg or "wg)
another common name for the inch 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.
inch pound (in·lbf or in lb)
a traditional unit of work or energy, equal to 1/12 foot pound, about 0.112 985 joule or 1.0709 x 10-4 Btu.
in d.
abbreviation for the Latin in die, daily, a unit of frequency used in medical prescriptions.
inhour (ih or inhr)
a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. In a reactor, fast-moving neutrons break apart atoms of uranium or plutonium; the fission of these atoms releases additional neutrons which keep the reaction going. The ratio R between the number of neutrons created and the number consumed in each cycle of fission must be very close to 1 in order for the reaction to be controlled. The reactivity is the difference k = R - 1 between this ratio and 1. One inhour is the reactivity which will cause the number of neutrons to increase by a factor of e = 2.71828 in one hour; a reactivity of t inhours will cause the number of neutrons to increase by a factor of e in 1/t hours. The exact size of the unit varies according to the design of the reactor. Enrico Fermi (1901-1954), the Italian-American physicist who built the first nuclear reactor, introduced this unit in 1947; its name is an acronym for "inverse hours." Other reactivity measures include the dollar and the milli-k.
international foot
the current foot unit of the English-speaking countries, equal to exactly 30.48 centimeters. See survey foot for additional information.
international nautical mile
the nautical mile as currently defined by international agreement, equal to exactly 1852 meters or 6076.11549 feet. This long name is sometimes used to distinguish the current nautical mile from older units.
international rubber hardness degree (IRHD)
a unit used to measure the hardness of rubber and similar materials (technically known as elastomers). Measurements are made using an IRHD durometer, and the results are usually similar, but not identical, to readings made with the older Shore "A" durometer. The International Organization for Standardization (ISO) and the American Society for Testing and Materials (ASTM), among other standards agencies, have published IRHD test procedures.
international unit (IU)
a unit used to measure the activity (that is, the effect) of many vitamins and drugs. For each substance to which this unit applies, there is an international agreement specifying the biological effect expected with a dose of 1 IU. Other quantities of the substance are then expressed as multiples of this standard. Examples: 1 IU represents 45.5 micrograms of a standard preparation of insulin or 0.6 microgram of a standard preparation of penicillin. Consumers most often see IU's on the labels of vitamin packages: in standard preparations the equivalent of 1 IU is 0.3 microgram (0.0003 mg) for vitamin A, 50 micrograms (0.05 mg) for vitamin C, 25 nanograms (0.000 025 mg) for vitamin D, and 2/3 milligram for (natural) vitamin E. Please note: for many substances there is no definite conversion between international units and mass units (such as milligrams). This is because preparations of those substances vary in activity, so that the effect per milligram of one preparation is different from that of another.
Internet time
a global decimal time system proposed by the Swatch Corporation. See beat for more information.
an abbreviation for inches per second (in/s), a traditional unit of velocity equal to 2.54 centimeters per second.
an abbreviation for inches per year (in/yr), a traditional unit for corrosion rates.
Irish acre
a traditional unit of land area in Ireland, equal to 160 square Irish perches (see next entry). This is equivalent in English units to 7840 square yards, 70 560 square feet, or about 1.6198 English acre (0.6555 hectare). The colpa, a traditional Irish unit of pasturage, is approximately equal to the Irish acre.
Irish mile
the traditional mile in Ireland is 6720 feet, which is 1.272 727 English mile or 2.048 256 kilometers. The discrepancy arose because the Irish perch, or rod, was standardized at 21 feet instead of the English figure of 16.5 feet. Just as in England, the Irish chain was equal to 4 perches (84 feet instead of 66 feet), the Irish furlong was equal to 10 chains (840 feet instead of 660 feet) and the mile was equal to 8 furlongs.
a traditional unit measuring the thickness of leather used in making shoes, especially the soles of the shoes. One iron is equal to 1/48 inch (0.5292 millimeter), so a sole 1/4 inch thick is described as "12 iron." The origin of this unit is unclear; comments from knowledgeable readers would be welcome!
the "short name" of the International Organization for Standardization. Based in Switzerland, the ISO sets international industrial standards, including standards for the use of units of measurements. For ISO film ratings, see ASA.
IT calorie
common name for the international steam table calorie, a unit of energy equal to exactly 4.1868 joules. See calorie.


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September 17, 2001