Precision unit conversion
The troy ounce is about 10% heavier than the ounce on your kitchen scale — here's where it comes from and why it still matters for pricing bullion and scrap gold.
If you've ever weighed a piece of gold jewellery on a kitchen scale and then tried to work out its value from the spot gold price, you may have ended up with a number that looked too low. The most likely reason: spot prices are quoted per troy ounce, while kitchen and postal scales almost always read in standard (avoirdupois) ounces or grams — and the two ounces aren't the same weight.
The troy weight system dates back to medieval European trade fairs (the name is generally linked to Troyes, a trading town in France) and became the standard for weighing precious metals, gemstones and, for a time, gunpowder. It was never replaced for bullion trading even after most countries moved to metric or the avoirdupois system for everyday goods — so gold, silver, platinum and palladium are still priced and traded in troy ounces worldwide today.
| Convert | Result |
|---|---|
| 1 troy ounce | 31.1035 grams |
| 1 standard (avoirdupois) ounce | 28.3495 grams |
| Difference | troy oz is about 9.7% heavier |
| 1 troy pound | 373.242g (12 troy oz — not 16) |
That last point catches people out too — a troy pound has only 12 troy ounces in it, not 16 like a standard pound, because the troy and avoirdupois systems built their larger units differently even though both call the base unit an "ounce."
For weighing individual rings, chains or small scrap pieces, a full troy ounce is often too large a unit to be useful, which is why jewellers commonly use the pennyweight (dwt) — exactly 1/20 of a troy ounce, or about 1.555 grams. If you're selling scrap gold and the buyer quotes a price "per dwt", that's the unit they mean.
If you're pricing gold, silver or platinum against the spot price, always weigh in — or convert to — troy ounces first. Weighing on a standard kitchen scale in grams is fine, since grams convert cleanly to troy ounces (divide by 31.1035); the mistake is assuming a standard ounce and a troy ounce are interchangeable, which will undervalue the item by roughly 10%.
The troy weight system has been the standard for precious metals since medieval trading fairs and was never replaced, so bullion markets, refiners and jewellers worldwide still quote weights and prices in troy ounces.
31.1035 grams — about 9.7% heavier than a standard (avoirdupois) ounce at 28.3495 grams.
A pennyweight (dwt) is 1/20 of a troy ounce, or about 1.555 grams — a traditional unit still used by jewellers for weighing gold and small precious metal items.