What is a Grain?

The grain equals exactly 64.79891 milligrams (0.06479891 g) and is the smallest unit in the traditional avoirdupois weight system. Originally based on the average mass of a single grain of wheat or barley, the unit was standardized to its current value with the 1959 international yard and pound agreement. The grain is still used in pharmaceutical compounding (some older drug formulations specify grains, especially aspirin and codeine), arrow tips and ammunition (bullet weights are commonly given in grains — a 9 mm bullet is around 115 grains), and precious-metal weighing (one troy ounce = 480 grains). It relates to the ounce (437.5 grains = 1 oz avoirdupois), the pound (7,000 grains = 1 lb), and the milligram (1 grain ≈ 64.8 mg). The unit reflects medieval agriculture's role in shaping early measurement systems.

Visual reference for how the grain relates to other weight units. Each row links to the full converter for that pair.

• Bullet and gunpowder mass in ballistics
• Older pharmaceutical dosing in Anglo-Saxon countries
• Arrow shaft and archery equipment weights

• ✓1 grain = 64.79891 mg exactly. 7000 grains = 1 pound.
• ✓Ballistics is one of the few modern contexts where grains are essential.
• ✓Some older pharmacology dosing survives in grain units — 5 gr ≈ 325 mg.

• ✗Confusing the grain (mass) with the troy grain in older precious-metal contexts — they are identical (64.8 mg).
• ✗Assuming "grain" refers to an amount of cereal — in firearms it is always a weight.
• ✗Reading "5 gr" as 5 grams — it is 5 grains (324 mg).