Grain (unit) - Wikipedia

Grains are commonly used to measure the mass of bullets and propellants.[11][12] In archery, the grain is the standard unit used to weigh arrows.[13]

In North America, the hardness of water is often measured in grains per U.S. gallon (gpg) of calcium carbonate equivalents.[14][15] Otherwise, water hardness is measured in the dimensionless unit of parts per million (ppm), numerically equivalent to concentration measured in milligrams per litre.[14][15] One grain per U.S. gallon is approximately 17.1 ppm.[14][note 1] Soft water contains 1–4 gpg of calcium carbonate equivalents, while hard water contains 11–20 gpg.[15]

Though no longer recommended, in the U.S., grains are still used occasionally in medicine as part of the apothecaries' system, especially in prescriptions for older medicines such as aspirin or phenobarbital.[16][17] For example, the dosage of a standard 325 mg tablet of aspirin is sometimes given as 5 grains.[16][18] In that example the grain is approximated to 65 mg, though the grain can also be approximated to 60 mg, depending on the medication and manufacturer.[16][19] The apothecaries' system has its own system of notation, in which the units symbol or abbreviation is followed by the quantity in lower case Roman numerals.[17][19][20] 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).[17][19][20][21]: 263  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],[21]: 34 [22]: 8  c̄ for "with",[21]: 56 [22]: 14  and cod. for codeine).[21]: 70 [22]: 19  The apothecaries' system has gradually been replaced by the metric system, and the use of the grain in prescriptions is now rare.[19]

In the U.S., particulate emission levels, used to monitor and regulate pollution, are sometimes measured in grains per cubic foot instead of the more usual ppm by volume.[23][24] This is the same unit commonly used to measure the amount of moisture in the air, also known as the absolute humidity.[25] The SI unit used to measure particulate emissions and absolute humidity is mg/m3.[23][25] One grain per cubic foot is approximately 2288 mg/m3.[note 2]