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


u [1]
the SI symbol for the unified atomic mass unit, as defined in 1960 and accepted by both chemists and physicists.
u [2]
a common replacement for the Greek letter µ as a symbol for the micron or micrometer. The correct symbol for this unit is µm.
a common replacement for µ- as a symbol for the SI prefix micro- (10-6). The symbol is frequently seen in combinations such as ug for the microgram (µg). The use of u- was approved by the International Organization for Standardization in its standard ISO 2955, issued in 1974. The problem at that time was that many of the character sets in common use did not include the Greek letter µ. This is much less of a problem today, so ISO 2955 has been withdrawn and is no longer in effect. The message from all standards agencies now is: use µ-, not u-, for micro-.
U [1]
a commercial unit of thermal conductance (heat flow). The U factor, as it is also called, is the conductance through an insulator as measured in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per °F of temperature difference between the two sides of the material. The U factor is numerically equal to 1 divided by the R value.
U [2]
a unit of distance used to measure the height of the standard racks in which audio, video, or computer components are mounted. 1U is equal to 1.75 inches (44.45 millimeters), so that, for example, a 2U component is 3.5 inches high, and a 22U rack houses a stack of components 38.5 inches high.
U [3]
usual symbol for the enzyme unit.
U [4]
German symbol for a turn or revolution (Umdrehung), usually seen in combinations such as U/min, the German equivalent of the English symbol rpm for revolutions per minute.
an alternate symbol for the astronomical unit (au).
an alternate symbol for the international unit (IU). In many languages, the two words of the phrase "international unit" are reversed. In French, for example, the phrase is unité international.
uld, ulp
units of data precision used in computer science. The symbol ulp stands for "unit in the last place," the smallest increment in a variable that can be recorded internally by the machine. Similarly, uld stands for "unit in the last digit," a change by 1 in the last (right-most) digit of data represented decimally.
um, ums
symbols sometimes used for the micrometer (micron). The symbol um is acceptable in situations where the Greek letter mu (µ) is not available to make the proper symbol µm. However, the "plural" symbol ums is never acceptable, because the SI prohibits adding -s to a symbol to form a plural.
uncia [1]
a Latin name for the fraction 1/12, subsequently used in many ways to represent a twelfth part. The Romans had no mathematical notation for fractions. When they needed to refer to a fractional part of anything, they would often state its nearest equivalent in unciae.
uncia [2]
the Roman ounce, equal to about 27.2875 grams or 0.9625 ounce avoirdupois. There were 12 ounces in a Roman pound, and the word uncia means a 12th part; its name gives us both the ounce and the inch.
uncia [3]
the Roman inch, equal to about 2.473 centimeters or 0.9734 English inch.
unit [1]
when counting, the word "unit" means "one." For example, if a car dealer expects a shipment of 20 units, that means 20 cars.
unit [2]
a shorthand word for a variety of named "units," particularly the international unit in pharmacology.
unit [3] (of blood)
a unit of volume for human blood and various blood components or products. A unit of whole blood is 450 milliliters, which is about 0.9510 U.S. pint. For components of blood, one unit is the amount of that substance that would normally be found in one unit of whole blood. The adult human body contains roughly 12 units of whole blood. Note: although the unit is defined by volume, it is actually measured by mass at blood collection centers. Since the density of human blood is 1.053 kilograms per liter, the mass of a unit of blood is about 474 grams or 16.7 ounces.
unit [4] (of heroin)
a unit of mass or weight for heroin equal to exactly 700 grams (0.7 kilogram or about 1.543 pounds). This unit, standard in the southeast Asian drug trade, is also called the Asian unit in U.S. drug enforcement.
unit [5] (of alcohol)
a unit of the alcohol content of beverages, used primarily in Britain. One unit of alcohol represents an alcohol content of 10 milliliters or, by weight, 8 grams. The alcohol content of a liter of a beverage is numerically equal to the percentage of alcohol by volume, so that, for example, a wine that is 12% by volume contains 12 units per liter, or 9 units in a standard 750 ml (3/4 liter) bottle.
unit [6] (of energy)
in Britain, another name for the kilowatt hour, which was formerly called the Board of Trade unit.
unit call (UC)
a measure of telecommunications traffic density. The unit call is a dimensionless "unit" representing a traffic density of 100 call-seconds per hour, or 1/36 erlang.
unit case
a conventional unit of sales volume in the U.S. soft drink industry. A unit case consists of soft drinks, syrup, powder, or whatever equivalent to 24 eight-ounce servings (6 quarts or about 5.678 liters).
unit (magnetic) pole
a CGS unit measuring the strength of a magnetic pole. A unit magnetic pole repels an identical pole at a distance of one centimeter in a vacuum with a force of one dyne. The unit magnetic pole equals about 125.6637 nanowebers (nWb) or 12.566 37 maxwells (Mx).
Universal Time (UT or Z)
the correct name for the time system previously called Greenwich Mean Time (GMT): the standard time at longitude 0°. Universal Time is five hours later than Eastern Standard Time in the U.S. It is always stated on the 24-hour clock; thus an event occurring at 1:26:15 pm Eastern Standard Time occurs at 18:26:15 UT (five hours after 13:26:15). Technically, the time shown on clocks is figured from Coordinated Universal Time (UTC), an international system of time measurement regulated by very precise atomic clocks. From time to time (either on June 30 or December 31), UTC is adjusted by the addition of a "leap second". This event, widely reported in the press but poorly understood by the public, keeps UTC within 0.9 seconds of mean solar time at longitude 0° as measured by the Earth's slightly uneven rotation. See day [2] for additional information.
a traditional German weight unit, corresponding to the English ounce. The unze equals 1/16 pfund. Although the pfund has been assimilated into the metric system (as 500 grams), the unze is effectively obsolete. It varied in size from about 28 to 35 grams. The word comes directly from the Latin uncia; the plural is unzen.
a Roman unit of volume equal to 4 congii, 24 sextarii, or 1/2 amphora. This is equivalent to about 12.75 liters (3.37 U.S. liquid gallons or 2.80 British Imperial gallons). The Latin word urna was also used more broadly to mean a jug, giving rise to the English word urn.
U.S. shipping ton
the freight ton [5]; see also shipping ton.
USP unit
a unit used in the United States to measure the potency of a vitamin or drug, that is, its expected biological effects. For each substance to which this unit applies, the U. S. Food and Drug Administration has determined the biological effect associated with a dose of 1 USP unit. Other quantities of the substance can then be expressed in terms of this standard unit. In most cases, the USP unit is equal to the international unit (IU). "USP" is a registered trademark of the United States Pharmacopeial Convention, Inc., a private standards organization that establishes standards for the pharmaceutical industry.


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October 5, 2004