A grain is a unit of measurement of mass that is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever having used actual seeds or cereal grains. 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
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
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.:C-19 1 gram is approximately 15.43236 grains.: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 1⁄4 of a carat, or 50 mg (~ 0.7716 gr). The grain was also the name of a traditional French unit equal to 53.115 mg.
Current usage 
The grain is commonly used to measure the mass of bullets and gun propellants. The term also refers to a single particle of gunpowder, the size of which varies according to requirements. 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 US gallon (gpg) of calcium carbonate equivalents. Otherwise, water hardness is measured in the metric unit parts per million (ppm), equivalent to mg/L. One grain per US gallon is approximately 17.1 ppm.[note] Soft water contains 1–4 gpg of calcium carbonate equivalents, while hard water contains 11–20 gpg.
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. For example, the dosage of a standard 325 mg tablet of aspirin is sometimes given as 5 grains. 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. 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. 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).: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,:34:8 c̄ for "with",:56:14 and cod. for codeine).:70:19 The apothecaries' system has gradually been replaced by the metric system, and the use of the grain in prescriptions is now rare.
Particulate emission levels, used to monitor and regulate pollution, are commonly measured in grains per cubic foot. 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/m3. One grain per cubic foot is approximately 2,288 mg/m3.[note]
|carob seed||~200 mg|
|barley grain||~65 mg|
|wheat grain||~50 mg|
At least since antiquity, grains of wheat or barley were used by Mediterranean traders to define units of mass; along with other seeds, especially those of the carob tree. According to a longstanding tradition, 1 carat (the mass of a carob seed) was equivalent to the weight of 4 wheat grains or 3 barleycorns.:95 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.:120-1
The history of the modern British grain can be traced back to a royal decree in 13th 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.
By consent of the whole Realm the King's Measure was made, so that an English Penny, which is called the Sterling, round without clipping, shall weigh Thirty-two Grains of Wheat dry in the midst of the Ear; Twenty pennies make an Ounce; and Twelve Ounces make a Pound.
The pound in question is the Tower pound. The Tower pound, abolished in 1527, consisted of 12 ounces like the troy pound, but was 1⁄16 lighter. The weight of the original sterling pennies was 22½ troy grains, or 32 "Tower grains".:116
The troy pound was only "the pound of Pence, Spices, Confections, as of Electuaries", as such goods might be measured by a troi or small balance. The old troy standard was set by King Offa's currency reform, was in full use in 1284 (Assize of Weights and Measures, King Edward I), but was restricted to currency (the pound of pennies) until it was abolished in 1527. This pound was progressively replaced by a new pound, based on the weight of 120 gold dirhems of 48 grains. The new pound used a barley-corn grain, rather than the wheat grain.
Avoirdupois (goods of weight) refers to those things measured by the lesser but quicker balances: the bismar or uncel, the Roman balance, and the steelyard. The original mercantile pound of 25 shillings or 15 (tower) ounces was displaced by variously the pound of the Hanseatic League (16 tower ounces) and by the pound of the then-important wool trade (16 ounces of 437 grains). A new pound of 7680 grains was inadvertently created by as 16 troy ounces, referring to the new troy rather than the old troy. Eventually, the wool pound won out.
The avoirdupois pound was defined in prototype, rated as 6992 to 7004 grains. In the Imperial Weights and Measures Act of 1824, the avoirdupois pound was defined as 7000 grains exactly. The act of 1855 that authorised Miller's new standards to replace those lost in the fire that destroyed the Houses of Parliament. The standard was an avoirdupoise pound, the grain being defined as 1/7000 of it.
The division of the carat into four grains, survives in both senses well into the early twentieth century. For pearls and diamonds, weight is quoted in carats, divided into four grains. The carat was eventually set to 205 milligrams (1877), and later 200 milligrams. For touch or fineness of gold, the fraction of gold was given as a weight, the total being a solidus of 24 carats or 96 grains.
See also 
- 1.^ The exact value of one grain per US gallon in mg/L (ppm) is 64.79891 ÷ 3.785411784.:C-12,C-14
- 2.^ The exact value of one grain per cubic foot in mg/m3 is 64.79891 ÷ 0.028316846592.:C-10,C-14
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