The grain, exactly
The grain is the smallest unit of the old English weight systems, and it is defined exactly. NIST Handbook 44, Appendix C, gives 1 grain as 64.798 91 milligrams, and marks the figure as exact. The UK Weights and Measures Act 1985, Schedule 1, reaches the same value by a different route: it defines the grain as 1/7000 of the pound, and the pound as 0.453 592 37 kilogram. Dividing one by the other gives 64.798 91 mg with nothing left over.
That makes the grain one of the few traditional units with an exact metric value. One gram is about 15.432 grains, and one grain is about 0.0648 g.
One grain in three systems
Avoirdupois weight (the everyday pound and ounce), troy weight (precious metals) and apothecaries’ weight (historic pharmacy) have different ounces and pounds, but they share the grain. NIST’s tables state this under each system: the grain is an equivalent quantity in avoirdupois, troy and apothecaries units of mass.
What differs is how many grains each larger unit holds. The avoirdupois pound is 7,000 grains and its ounce 437.5. The troy ounce is 480 grains, and the troy pound, 12 troy ounces, is 5,760. The apothecaries’ ounce is also 480 grains, the same as the troy ounce, and the UK Act defines it that way. The pennyweight, a troy unit, is 24 grains.
Because they share the grain, every one of these units converts exactly through it. That is how this converter works: everything passes through grams, and each unit is a fixed multiple of the grain.
Where grains turn up
In jewellery and metalwork the grain appears mostly in older records: antique scale weights, historical coinage and assay specifications, and old catalogues. Outside jewellery it is still the usual unit for bullets and powder charges, and it survives in some pharmaceutical labelling.
In the UK the Weights and Measures Act lists the grain in Part VI of Schedule 1. Units in that part may not be used for trade except as supplementary indications, so a UK trader states the weight in grams and may give grains alongside.
Not the pearl grain
Pearls have their own “grain”, and it is a different unit. The pearl grain is a quarter of a metric carat, 50 milligrams, a trade unit for weighing natural pearls. The metal grain is 64.798 91 mg, about 30% heavier. A historic pearl weighed at 100 grains is 5 grams; 100 metal grains would be 6.48 grams.
A weight written only as “grains” needs reading in context. If it describes a pearl, it is almost certainly the pearl grain, and the pearl weight converter handles it. If it describes metal, a coin or a scale weight, it is the grain on this page.
Grains, carats and gemstones
The metric carat used for gemstones is 200 mg exactly, so one grain is 0.323 994 55 carats and one carat is about 3.086 grains, a relationship NIST lists among its equivalents. The carat appears in this converter so that a gemstone weight can be compared with a metal weight, although the two are seldom used together.
Precision
Every figure here is exact by definition, so the only rounding is in the display. The result is only as good as the weight entered, and a few grains is a very small mass: one grain is less than a fifteenth of a gram. Weighing anything in grains with confidence takes a scale that reads to a milligram or two.