According to the Wikipedia entry: A grain is a unit of measurement of mass, and in the troy weight, avoirdupois, and Apothecaries' system, equal to exactly 64.79891 milligrams. It is nominally based upon the mass of a single virtual 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. Rather, expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas, essentially the premodern equivalent of legal boilerplate.:27 Another source states that it was defined as the weight needed for 252.458 units to balance a cubic inch of distilled water at 30 inches of mercury pressure and 62 degrees Fahrenheit for both the air and water. 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 of mass.:C-6 The unit was based on the weight of a single grain of barley, considered equivalent to 1 1⁄3 grains of wheat.:95 The fundamental unit of the pre-1527 English weight system known as Tower weights, was a different sort of grain known as the "wheat grain". The Tower wheat grain was defined as exactly 45⁄64 of a troy grain.:74 My powder measure is calibrated in grains based on the mass of that volume of 2fg GOEX Black Powder.