Usage in North America
A box of .38 Special cartridges that have 148-grain bullets.
A box of .38 Special cartridges that have 148-grain bullets.

Grains are currently used in the United States and Canada to measure the mass of bullets and gunpowder, and scales for handloading measure in grains; bullets are generally measured in increments of 1 grain, gunpowder in increments of 0.1 grains.[3] Grains are used to weigh the tools involved in the sport of fencing, including the foil. Grains are used to measure arrows, and arrow parts in archery.
The 5-grain aspirin. The back of a bottle of aspirin indicating that the dosage is "325 mg (5gr)"
The 5-grain aspirin. The back of a bottle of aspirin indicating that the dosage is "325 mg (5gr)"

Grains are still used occasionally in medicine in the United States, especially in medical prescriptions, usually via the abbreviation "gr." For example, a regular tablet of aspirin is sometimes referred to as "five grain aspirin," or 325 mg. Grains are commonly used for medications that have been included in the United States Pharmacopeia for many decades, such as codeine, opium and phenobarbital combinations. For example, a prescription for tablets containing 325 mg of aspirin and 30 mg of codeine (brand name is Empirin with codeine), is written thus: "ASA gr. v c cod. gr. ss tablets," where "ASA" is short for aspirin (AcetylSalicylic Acid), "v" is the roman numeral for five, "c" is the abbreviation for "with" and "ss" stands for one-half. Likewise, a prescription for B&O Supprettes #15A, which is a compound medication containing belladonna alkaloids and opium, may be written: "Belladonna gr. 1/4 c opium gr ss", as B&O Supprettes #15A contain 16.2 mg (1/4 grain) of powdered belladonna and 30 mg (1/2 grain) of opium. Similarly, a prescription for 60 mg (1 grain) of phenobarbital is often written: "Phenobarb. gr. i". Formulations of these older medications (e.g., Donnatal, Phenobarbital, etc.), often use grains on the product label along with the metric equivalent. For example, Extended-Release Donnatal tablets contain 3⁄4 grain (approximately 48.6 mg) of phenobarbital. See, for example: Donnatal Extentabs Prescribing Information. See also Phenobarbital tablets with grain equivalents in veterinary medicine. Given the potential error in mistaking the abbreviations for "grains" and "grams" (gr and g, respectively), and for consistency with other medical orders, metric units are preferred to avoirdupois or apothecary units; hence, the use of grains in the medical profession is rapidly becoming outmoded.
Bottle of 1/4 grain phenobarbital tablets
Bottle of 1/4 grain phenobarbital tablets

Grains are also used in environmental permitting to quantify particulate emissions. Grains are used to measure the amount of moisture per cubic foot of air, a measure of absolute humidity.[4]

[edit] History
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.[5] But 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.[6]

The history of the modern troy grain can be traced back to a royal decree in 13th century England:

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-pence make an Ounce; and Twelve Ounces make a Pound.
—Henry III of England[6]

The traditional reading of this text is that it refers to the troy pound, and that the reference to sterling pennies is purely symbolic. According to a more recent reading, however, the pound in question is the Tower pound, and it talks about the actual mass of real sterling pennies.. The Tower pound, abolished in 1527, consisted of 12 ounces like the troy pound, but was 1⁄16 lighter. In any case, with both readings one needs to substitute 24 barley grains for the 32 wheat grains of the text, according to the general convention of a 4:3 equivalence, for it to make sense. The weight of the original sterling pennies was 22˝ troy grains, or 24 "Tower grains" if the Tower pound was divided in the same way as the troy pound.[6] Regardless of which pound this text originally referred to, a (troy) ounce still equals 20×24 = 480(troy) grains, and a pound consists of 12×20×24 = 5760 grains.

Originally the troy pound was only "the pound of Pence, Spices, Confections, as of Electuaries", and the merchants used different standards, which had to be compatible with those used abroad.[6] One such standard, the avoirdupois pound, was later fixed officially at exactly 7000 troy grains. It consists of 16 avoirdupois ounces of 437˝ troy grains each.[