Two ounces, one name, different weight
| Unit | Grams | Grains | Ounces per pound |
|---|---|---|---|
| Troy ounce | 31.1034768 | 480 | 12 |
| Avoirdupois ounce | 28.349523125 | 437.5 | 16 |
Both systems share the grain as their smallest unit — 480 grains to the troy ounce, 437.5 to the avoirdupois ounce — which is how the two systems were reconciled when the pound was standardised. The grain itself is ancient, supposedly the weight of a single grain of barley, and it survives today in ammunition and archery as well as precious metals.
Why metals kept their own ounce
Everyday goods moved to the avoirdupois system because it suited bulk trade: 16 ounces to the pound divides neatly into halves, quarters and eighths. Precious metals stayed troy because their trade was built on the Troyes fairs' weights, and because the troy system's finer subdivisions — pennyweights of 24 grains, grains themselves — suited valuing small, expensive quantities. When the imperial system was standardised in the 19th century, the troy pound was abolished for most purposes but the troy ounce was explicitly kept for precious metals, and the London bullion market never had a reason to change.
The mistake that costs 10%
A gold coin weighing "one ounce" holds one troy ounce, 31.1035 grams — not 28.35 grams. At a spot price of, say, £2,000 per troy ounce, valuing a 10-ounce holding with avoirdupois ounces understates it by roughly 2.75 grams per ounce, or about £1,770 on the holding. The reverse error — weighing scrap gold on a kitchen scale in avoirdupois ounces and multiplying by the spot price — overstates what a dealer will pay, since the dealer weighs in troy. Know which ounce your scale uses before you trust either number.
For the everyday ounce behind kitchen measures, see how many grams are in an ounce, and for the pound behind both systems see how many ounces are in a pound.