Grain (unit)

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A grain is a unit of measurement of mass, and in the troy weight, avoirdupois, and apothecaries' systems, equal to exactly 64.79891 milligrams. It is nominally based upon the mass of a single ideal seed of a cereal. From the Bronze Age into the Renaissance, the average masses of wheat and barley grains were part of the legal definitions of units of mass. Expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas. Another source states that it was defined such that 252.458 units would balance 1 cubic inch (16 cm3) of distilled water at an ambient air-water pressure and temperature of 30 inches of mercury (100 kPa) and 62 °F (17 °C) respectively. Another book states that Captain Henry Kater, of the British Standards Commission, arrived at this value experimentally. The grain was the legal foundation of traditional English weight systems, and is the only unit that is equal throughout the troy, avoirdupois, and apothecaries' systems...

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