One grain is exactly 0.06479891 grams, or 64.79891 milligrams. It is the oldest unit still in regular use in the English-speaking world, and it is the one point where the avoirdupois, troy and apothecaries' systems all agree.
About the units
The grain was originally the mass of a single barleycorn or wheat grain, and it appears in English weights legislation from the medieval period onwards. Its survival is due to that shared definition across three otherwise incompatible systems: 7,000 grains make an avoirdupois pound, 5,760 make a troy pound, and 5,760 make an apothecaries' pound. When the international agreement of 1959 fixed the avoirdupois pound at 0.45359237 kg, the grain followed automatically at 0.45359237 ÷ 7,000 = 0.00006479891 kg. Every other grain-based value in every system inherits that exactness.
The exact factor
The exact value is 64.79891 mg. Practical reference points: a troy ounce is 480 grains, an avoirdupois ounce is 437.5 grains, and a carat is 3.0865 grains. In shooting sports the grain is used for both bullet mass and powder charge, which are entirely different magnitudes on the same scale — a 55-grain bullet is 3.56 g, while a 25-grain powder charge is 1.62 g. Reloading data is published in grains to a tenth, which is 6.5 mg, and that resolution is meaningful for pressure.
Where you meet this conversion
Ammunition and reloading are the dominant modern use, and the grain is universal there even in fully metric countries: European ammunition boxes list bullet mass in grains alongside grams. Archery uses it for arrow and point mass. Pharmacy used it historically, and it lingers in a few legacy drug strengths — aspirin at 5 grains, 325 mg, is why the American standard tablet is that odd number rather than a round 300. Dentistry and some water-hardness measurements in the United States also retain it.