A grain is a unit of mass equal to 0.06479891 gram, in both the avoirdupois and troy weight systems. An avoirdupois pound is 7000 grains, whereas a troy pound is 5760 grains.

Grains are currently used to measure the mass of bullets and gunpowder, and for older drugs such as aspirin.

A metric or pearl grain is equal to 50 milligrams or 1/4 of a carat as used in pearl and diamond nomenclature.

Originally, a grain was the weight of a grain seed from the middle of an ear of wheat.