grain
Massexact by definition
The grain is exactly 1/7000 of an avoirdupois pound, which is exactly 0.00006479891 kilograms, or 64.79891 milligrams. The value is exact because the pound is exact. It is the one unit common to the avoirdupois, troy and apothecaries' systems, since all three were built on it.
Watch out: The grain is still the working unit in several trades and is easy to mistake for something else. Bullet and arrow masses and powder charges are quoted in grains, so a 55 grain bullet is 3.56 g. Older pharmacy dosing used it too, and the standard 5 grain aspirin tablet is the origin of today's 325 mg tablet. HVAC psychrometrics uses grains of moisture per pound of dry air, which is a humidity ratio and not a weight of anything you can hold.
| 1 gr | 0.00006479891 kg |
The grain was originally the mass of a single grain of barley taken from the middle of the ear, one of the oldest surviving units of weight in the English-speaking world.