Grain

What the unit was used for

The grain is the smallest unit of the English system and the only one shared by all three weight systems, avoirdupois, troy and apothecaries.

That shared base is what makes a troy ounce and an avoirdupois ounce comparable at all.

Its notional origin is the same barleycorn that Edward I’s statute used for the inch, and it is just as rhetorical, since real grains vary.

The law fixed it instead as one seven-thousandth of the avoirdupois pound, and Schedule 1 to the Weights and Measures Act 1985 still gives it that way.

It remains in real use for bullets and propellant charges, for arrow and bowstring weights, and it ruled pharmacy until the apothecaries’ system was abolished in 1976.

In the system

1 grain = 1/7000 avoirdupois pound = 1/5760 troy pound