Grain (unit)

The small golden disk close to the 5 cm marker is a piece of pure gold weighing one troy grain. Shown for comparison is a tape measure and coins of major world currencies.

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.[1]:27[2] 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.[3] Another book states that Captain Henry Kater, of the British Standards Commission, arrived at this value experimentally.[4]

The grain was the legal foundation of traditional English weight systems,[5] and is the only unit that is equal throughout the troy, avoirdupois, and apothecaries' systems of mass.[6]:C-6 The unit was based on the weight of a single grain of barley, considered equivalent to ​1 13 grains of wheat.[5][7]: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".[8] The Tower wheat grain was defined as exactly ​4564 of a troy grain.[1]:74

Since the implementation of the international yard and pound agreement of 1 July 1959, the grain or troy grain (symbol: gr) measure has been defined in terms of units of mass in the International System of Units as precisely 64.79891 milligrams.[6]:C-19[9] 1 gram is approximately 15.43236 grains.[6]:C-13 The unit formerly used by jewellers to measure pearls, diamonds, or other precious stones, called the jeweller's grain or pearl grain, is equal to ​14 of a carat, or 50 mg (~ 0.7716 gr).[5] The grain was also the name of a traditional French unit equal to 53.115 mg.[5]

In both British Imperial and U.S. customary units, there are precisely 7,000 grains per avoirdupois pound, and 5,760 grains per troy pound or apothecaries pound.[6]:C-6–C-7

Current usage

A box of .38 Special cartridges that have 148-grain bullets

The grain is commonly used to measure the mass of bullets and propellants.[10][11] The term also refers to a single particle of gunpowder, the size of which varies according to requirements.[12] In archery, the grain is the standard unit used to weigh arrows.[13]

In dentistry, gold foil, used as a material to restore teeth,[14] is measured in grains.[15][16]

In North America, the hardness of water is often measured in grains per US gallon (gpg) of calcium carbonate equivalents.[17][18] Otherwise, water hardness is measured in the metric unit parts per million (ppm), equivalent to mg/L.[17][18] One grain per US gallon is approximately 17.1 ppm.[17][note] Soft water contains 1–4 gpg of calcium carbonate equivalents, while hard water contains 11–20 gpg.[18]

The 5-grain aspirin. The usage guidance label on a bottle of aspirin indicates that the dosage is "325 mg (5 gr)".

Though no longer recommended, grains are still used occasionally in medicine as part of the apothecaries' system, especially in prescriptions for older medicines such as aspirin or phenobarbital.[19][20] For example, the dosage of a standard 325 mg tablet of aspirin is sometimes given as 5 grains.[19][21] 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.[19][22] The apothecaries system has its own system of notation, in which the unit's symbol or abbreviation is followed by the quantity in lower case Roman numerals.[20][22][23] 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).[20][22][23][24]: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 aspirin,[24]:34[25]:8 c̄ for "with",[24]:56[25]:14 and cod. for codeine).[24]:70[25]:19 The apothecaries' system has gradually been replaced by the metric system, and the use of the grain in prescriptions is now rare.[22]

Particulate emission levels, used to monitor and regulate pollution, are commonly measured in grains per cubic foot.[26][27] This is the same unit commonly used to measure the amount of moisture in the air, also known as the absolute humidity.[28] The SI unit used to measure particulate emissions and absolute humidity is mg/m3.[26][28] One grain per cubic foot is approximately 2288 mg/m3.[note]

Other Languages
asturianu: Granu (unidá)
беларуская: Гран
беларуская (тарашкевіца)‎: Гран
čeština: Grain
eesti: Graan
فارسی: دانه (یکا)
français: Grain (unité)
Ido: Grano
lietuvių: Granas
Nederlands: Grein (gewicht)
日本語: グレーン
norsk nynorsk: Nürenberggran
português: Grão (massa)
русский: Гран
slovenščina: Gran
suomi: Graani
svenska: Grain
українська: Гран (одиниця)
中文: 格令