A grain is a unit of measurement of mass, and in the troy weight, avoirdupois, and apothecaries' systems, equal to exactly . 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 of distilled water at an ambient air-water pressure and temperature of and 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 of mass. The tower "wheat" grain was defined as exactly (≈) of the troy "barley" grain. One gram is thus approximately equivalent to .
Volume
In United States customary units, the "grain of water" exists as a unit of volume, conventionally defined as the volume of a mass of water equal to 1 grain at a density of 1 . As such, 1 grain of water corresponds to . It is commonly used to measure the case capacity of cartridges.
Current usage
thumb|250px|A box of [[.38 Special () cartridges that have bullets]]
Grains are commonly used to measure the mass of bullets and propellants. In archery, the grain is the standard unit used to weigh arrows.
In North America, the hardness of water is often measured in grains per U.S. gallon () of calcium carbonate equivalents. Otherwise, water hardness is measured in the dimensionless unit of parts per million (), numerically equivalent to concentration measured in milligrams per litre. For example, the dosage of a standard tablet of aspirin is sometimes given as . In that example the grain is approximated to , though the grain can also be approximated to , depending on the medication and manufacturer. For amounts less than one, the quantity is written as a fraction, or for one half, ss (or variations such as ss., ṡṡ, or s̅s̅). Therefore, a prescription for tablets containing 325 mg of aspirin and 30 mg of codeine can be written "ASA gr. v c̄ cod. gr. ss tablets" (using the medical abbreviations ASA for acetylsalicylic acid [aspirin], c̄ for "with", This is the same unit commonly used to measure the amount of moisture in the air, also known as the absolute humidity. The SI unit used to measure particulate emissions and absolute humidity is mg/m. Since the weights of these seeds are highly variable, especially that of the cereals as a function of moisture, this is a convention more than an absolute law.
The history of the modern British grain can be traced back to a royal decree in thirteenth century England, re-iterating decrees that go back as far as King Offa (eighth century). The Tower pound was one of many monetary pounds of 240 silver pennies.