Troy ounce vs ounce

A troy ounce is a unit of mass equal to exactly 31.1034768 grams. It is the standard weight for precious metals: gold, silver, platinum, palladium, and gemstones are all traded by it worldwide.

It is built from the grain, an old unit that survives in this one corner of the metric world. A grain is exactly 64.79891 milligrams, and a troy ounce is 480 grains. That is where the long decimal comes from. It is not a measurement with error bars; it is a definition.

You will see it written ozt, oz t, or just "oz" in a context where troy is understood. A bullion dealer listing a "1 oz" gold bar means 31.1034768 g.

Exactly 31.1034768 grams. Rounded, that is 31.1 g. The figure is the same for gold, silver, or anything else: a troy ounce is a weight, so it does not change with the metal.

What does change is what that weight is worth, and how much of it is the metal you care about. A troy ounce of 24K gold is nearly all gold. A troy ounce of 14K gold holds only about 18.144 g of it, because 14K is 58.33% pure.

The troy ounce is heavier than the standard ounce. So a troy pound must be heavier too, surely? It is not. People get this backwards more than anything else on this page.

A troy pound holds only 12 troy ounces. An avoirdupois pound holds 16 standard ounces. Twelve of the heavier unit still adds up to less than sixteen of the lighter one:

There is no notation rule to rely on. Dealers often write plain "oz" when they mean troy. Go by the material instead:

• Gold, silver, platinum, palladium, gemstones → troy. Spot prices, coin weights, bar weights, and jewellery weights are all troy. A "1 oz" gold coin holds 31.1034768 g of gold.
• Anything else → standard. Food, nutrition labels, packaging, body weight, postage, freight. A recipe never means troy.
• Silverware is a genuine edge case. Sterling silver is often sold by troy weight. But a kitchen scale reads avoirdupois. Weigh in grams and the question goes away.

When in doubt, work in grams. A gram means one thing only. That is why the metals trade now quotes grams next to troy ounces, and why our chart shows both columns instead of making you pick first.

The troy system is named after Troyes, a French market town. Its fairs drew merchants from across medieval Europe. Troy weight covered precious metals and coinage. Those had to weigh the same everywhere, because their weight was their value.

Avoirdupois means "goods of weight" in Old French. It covered ordinary trade: wool, grain, metals by the ton. Bulkier things, weighed less precisely, on scales built for commerce rather than coinage.

Britain's Weights and Measures Act of 1878 scrapped troy weight for general use. It kept the troy ounce for precious metals. That one exception is why, well over a century later, a gold price and a flour weight use different ounces.