Troy ounces explained

The short version: A troy ounce is 31.1 grams, about 10% heavier than the 28.3-gram ounce on a kitchen scale. Precious metal prices are always per troy ounce.

When you see “1 oz” on a gold coin or a price “per ounce,” it means a troy ounce, not the ounce you use for groceries.

The numbers

• 1 troy ounce = 31.103 grams
• 1 regular (avoirdupois) ounce = 28.350 grams
• A troy ounce is about 9.7% heavier than a regular ounce.
• 1 pennyweight (dwt) = 1.555 grams, or 1/20 of a troy ounce. Jewelers often weigh gold in pennyweights.
• 1 kilogram = 32.15 troy ounces

Why it matters

If you weigh a bracelet on a kitchen scale in ounces and multiply by the gold price, the answer will be wrong in two ways: the ounce is the wrong size, and the bracelet is not pure gold. Weigh in grams, divide by 31.103 to get troy ounces, then account for purity.

Copper is the exception

Copper rounds are often sold by the regular (avoirdupois) ounce, which is why our copper page lists weights as “oz (avdp).”