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abbr. kg, fundamental unit of mass in the metric systemmetric system,
system of weights and measures planned in France and adopted there in 1799; it has since been adopted by most of the technologically developed countries of the world.
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, defined as the mass of the International Prototype Kilogram, a platinum-iridium cylinder kept at Sèvres, France, near Paris. Copies of this standard are deposited at bureaus of standards throughout the world, and other units of mass are defined in terms of it. When the metric system was originally devised, the kilogram was defined so that 1,000 cubic centimeters (1 cubic decimeter) of pure water has a mass of exactly 1 kilogram.


(kil -ŏ-gram) Symbol: kg. The SI unit of mass. It is defined as the mass of a prototype platinum-iridium cylinder kept at the International Bureau of Weights and Measures at Sèvres, France. See also solar mass.



a unit of mass; one of the seven basic units of the International System of Units (SI). It is equal to the mass of the International Prototype Kilogram, kept at the International Bureau of Weights and Measures. Its abbreviated designations are as follows: Russian, kg; international, kg.

In the 18th century, when the metric unit system was first introduced, a kilogram was defined as the mass of 1 cubic decimeter (dm3) of water at 4°C (the temperature of highest density). However, the mass of the prototype kilogram (a cylindrical platinum weight made in 1799) was found to be about 0.028 g greater than the mass of 1 dm3 of water. In 1889 the currently existing definition of a kilogram was accepted, and a weight designated by the symbol Κ (German capital K) was approved as the International Prototype Kilogram. It is made from a platinum-iridium alloy (10 percent Ir) and has the shape of a cylinder 39 mm in diameter and 39 mm high. Of the 40 copies of this prototype that were made, two (nos. 12 and 26) were turned over to Russia. Standard no. 12 is accepted in the USSR as the primary government standard of mass; standard no. 26 is a copy of the primary standard.

For a long time no distinction was made between the mass and weight of bodies. Thus, a kilogram served as a unit not only of mass but also of weight (the force of gravity). Differentiation between units of mass and weight was established at the Third General Conference of Weights amd Measures (1901). A decision of the conference emphasized that the weight of a body is equal to the product of its mass and free-fall acceleration and introduced the concept of normal weight and normal gravitational acceleration (980.665 cm/sec2). A separate unit of force and weight—the kilogram-force—was established at that time. The same principle is preserved in the International System of Units, where the newton has been adopted as the unit for measurements of force.

Designations for multiples and fractions of a kilogram are formed by adding a prefix to the designation “gram,” such as megagram (Mg) and milligram (mg). Although a kilogram is not one of the units that can be defined by invariable natural constants (that is, the prototype standard for a kilogram is not reproducible), its accuracy satisfies the requirements of modern science and engineering (the relative error in comparisons with the prototype does not exceed 2 X 10-9).


Smirnova, N. A. Edinitsy izmerenii massy i vesa ν Mezhdunarodnoi sisteme edinits. Moscow, 1966.



The unit of mass in the meter-kilogram-second system, equal to the mass of the international prototype kilogram stored at Sèvres, France. Abbreviated kg.


The International Standard unit for mass; equals 1000 grams.


1. one thousand grams
2. the basic SI unit of mass, equal to the mass of the international prototype held by the Bureau International des Poids et Mesures. One kilogram is equivalent to 2.204 62 pounds.