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


the SI symbol for the volt (see below).
a common symbol for the voltage in an alternating current (AC) circuit (see var below for comments on alternating current). The SI does not allow symbols to be modified with additional information; instead of "12 VAC," write "AC 12 V."
a traditional unit of mass or weight in countries of the former Yugoslavia. Originally considered to be the weight that could be carried by a wagon, the unit has been "metrized" and is now defined to be equal to the dekatonne, that is, 10 metric tons or 22 046.23 pounds avoirdupois.
a symbol used in Europe, especially in Germany, for the equivalent weight.
a unit of the reactive electric power delivered by an alternating current (AC) circuit. In an AC circuit, the electric potential or voltage (measured in volts) and the current (in amperes) alternate direction, varying smoothly according to sine curves. In a purely resistive circuit, current is in phase with voltage. In a purely inductive circuit, the variations of the current would lag the variations in the voltage by 1/4 cycle, or 90°. In real circuits, the current can be separated into two parts: a part in phase with the voltage, and the "reactive" part, which lags the voltage by 90°. The reactive part does no net work; it simply heats the conductor. Reactive current does perform important magnetizing and voltage-regulation functions in real circuits. The reactive power is the product of the voltage and the reactive part of the current. The name of the unit is an acronym for volt-ampere-reactive.
a traditional unit of distance in Spanish- and Portuguese-speaking countries. The length of the vara varied (no pun intended), but in Spanish Latin America it was generally about 33 inches or a little longer. In Texas, where it was often used in land measurement, the vara was defined to equal exactly 33 1/3 inches, which is equivalent to 84.667 centimeters. In California, the vara was considered equal to 33 inches (83.82 centimeters) and in Mexico the former standard was 32.993 inches (83.802 centimeters). In southern South America the vara was usually about 34 inches (86.4 centimeters). The Spanish vara is shorter; it equals 32.908 inches or 83.587 centimeters. The Portuguese vara, on the other hand, is much longer; it equals 5 palmos or about 110 centimeters (43.3 inches). The word vara means a stick or pole.
a common symbol for the voltage in a direct current (DC) circuit. In DC circuits, both voltage and current are constant. The SI does not allow symbols to be modified with additional information; instead of "12 VDC," write "DC 12 V."
a traditional Russian unit of volume equal to 100 charki. The vedro is about 12.30 liters (3.249 U.S. liquid gallons or 2.706 British Imperial gallons). In Bulgaria, the vedro has also been used informally as a name for the dekaliter (exactly 10 liters or 2.642 U.S. liquid gallons). The word vedro means a bucket.
an old name for the yard, taken from the Latin word virga for a twig or stick. In modern French, verge is the customary word for the English yard.
a traditional unit of land area in the Channel Islands, a British territory just off the coast of France. The unit varied from one parish to another. In Jersey, a common estimate is that the vergee is equal to about 0.44 acre (about 0.178 hectare or 2130 square yards). In Guernsey it is somewhat smaller, about 0.4 acre (0.162 hectare or 1940 square yards). The name of the unit is a old Norman word meaning an orchard.
vershok or verchok
a traditional Russian unit of distance equal to 1/16 arshin, 1.75 inches or 4.445 centimeters. The plural is vershki.
verst, versta, vehrsta
a traditional Russian unit of distance, formerly used throughout eastern Europe. The verst equals 1500 arshin, which is 3500 feet, 0.662 88 mile, or 1066.8 meters.Although vehrsta is the best transliteration of the Russian, the spelling verst is common in English. The German spelling werst is also used sometimes. In Finnish the unit is called the virsta. The Russian plural is vehrsty.
Vickers hardness number (HV or VHN) or Vickers pyramid number (VPN)
a measure of the hardness of a metal introduced by Vickers in 1922. In the Vickers test (suitable for surface-hardened metals), a pyramidal diamond is pressed into the material being tested. The Vickers hardness is the amount of force applied to the diamond divided by the area of the indentation the diamond makes in the material; in practice the diagonal of the pyramidal indentation is measured and the result, computed in kilograms of force per square millimeter, is read from a table. (The reading should be stated as an empirical measurement, without units.) Up to about HV 500, the Vickers hardness is about 1.04 times the Brinell hardness.
viertel [1]
a traditional unit of volume in several European countries. Oddly, although the name means "quarter" in German the traditional viertel is not really 1/4 of any other unit. The Danish viertel equals 8 pots or about 7.74 liters (2.04 U.S. liquid gallons or 1.70 British Imperial gallons). In Switzerland the viertel is 40 schoppen, which is exactly 15 liters (3.9626 U.S. liquid gallons or 3.3000 British Imperial gallons).
viertel [2]
a unit of volume for wine in Austria, equal to exactly 1/4 liter (250 milliliters) or about 8.45 U.S. fluid ounces.
an obsolete unit of light intensity equal to 20.17 candelas. One violle is the intensity of a square centimeter of platinum, glowing at its melting temperature of 1769 °C (3216 °F). The unit is named for the French physicist Jules Violle (1841-1923), who proposed it in 1881; it was the first unit of light intensity that did not depend on the properties of a particular lamp.
an old English unit of land area equal to 1/4 hide. This is roughly 30 acres or 12 hectares. The virgate was also called the yardland or yard of land.
viscosity grade (VG)
a commercial rating of industrial lubricants. The grade numbers are approximately equal to the kinematic viscosity of the lubricant in centistokes. A table is provided showing the range of viscosities acceptable for each grade under the current standard of the International Organization for Standardization (ISO).
a term often used with a Snellen fraction in phrases such as "20/20 vision."
viss, vis, vise
a traditional unit of mass or weight in southern India and southeast Asia, equal to 100 ticals. The name is derived from the Tamil visai. The traditional size of the viss is about 1640 grams (about 3.62 pounds), but it is currently used in the Southeast Asian drug trade as a metric unit equal to exactly 1600 grams (1.6 kilograms or 3.527 pounds). This metric unit is called the choi or joi in Laos and Thailand.
volcanic explosivity index (VEI)
a measure of the severity of a volcanic eruption. The VEI scale is from 0 to 8; an index value of v corresponds to an output of at least 103+v cubic meters of magma. The Mt. St. Helens eruption of 1980 was a 5 on this scale, and the largest eruption of historic times (the Tambora eruption of 1815) rated a 7. A table is provided.
volt (V)
the SI unit of electric potential. Separating electric charges creates potential energy, which can be measured in energy units such as joules. Electric potential is defined as the amount of potential energy present per unit of charge. Electric potential is measured in volts, with one volt representing a potential of one joule per coulomb of charge. The name of the unit honors the Italian scientist Count Alessandro Volta (1745-1827), the inventor of the first battery. (Note: I am sometimes asked about standard line voltages in different countries. Steve Kropla has a helpful chart with this information.)
volt ampere (V·A)
a unit of electrical load used in power engineering. In an alternating current circuit, if the potential (measured in volts) and the current (measured in amperes) vary in phase with each other, then the power delivered (measured in watts) is the product of the potential and the current. In actual circuits, the potential and current are usually out of phase (see var, above), causing the device receiving the power to draw more current than its wattage requirements would suggest. The product of the potential and the actual current is the load, in volt amperes.
volume unit (vu)
a unit used in telecommunications to describe the volume of a radio or television signal carrying complex information such as speech or music. These waves vary wildly in amplitude in order to represent the message they carry, so it is difficult to describe mathematically how large their "average" amplitude is. To solve this problem, the American Standards Association decided in 1951 to establish as a reference a signal which generates 1 milliwatt of power in a circuit having an impedance of 600 ohms. The volume of an incoming signal, in volume units, is equal to the number of decibels by which it exceeds this reference level. Thus the volume unit is (in a complicated way) a variation of the decibel.
volumetric unit (vu)
a unit of volume equal to 200 cubic feet (5.663 cubic meters), used in the U.S. forest products industry for wood chips and other by-products of lumber production.
volumetric weight
a measure of the size of a package used for billing purposes in the airline industry. The volumetric weight of a package, in kilograms, is equal to lwh/6000, where l, w, and h are the maximum length, width, and height measurements of the package, in centimeters. Shippers are billed for the larger of the volumetric weight and the actual weight of the package. Since the calculation assumes a density of only 1/6 gram per cubic centimeter (1/6 the density of water), the volumetric weight is used only for rather light packages that take up a lot of space on the aircraft.
an abbreviation for "by volume," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. Thus 2% v/v means that the volume of the substance is 2% of the total volume of the solution or mixture. See also w/v.


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May 18, 2000