Troy ounce vs ounce: why gold is weighed differently

Precious metals are priced per troy ounce, which is 31.1035 grams. The ordinary ounce on kitchen scales, the avoirdupois ounce, is 28.3495 grams, so a troy ounce is about 9.7% heavier. Using the wrong ounce when converting a gold or silver price to grams makes the price per gram look about 10% higher than it is.

The two ounces

• Avoirdupois ounce (oz): 28.349523125 g, 1/16 of a pound. Used for food, post and everyday goods in the US and UK.
• Troy ounce (oz t): 31.1034768 g. Used for gold, silver, platinum and palladium in markets worldwide.

The Weight Converter lists the troy ounce and the carat separately from ordinary ounces, so you can convert between them and grams exactly.

Converting a metal price to grams

Divide the price per troy ounce by 31.1035. If gold is quoted at 2,500 per troy ounce, a gram costs 2,500 ÷ 31.1035 = 80.38. Dividing by 28.35 by mistake gives 88.19, about 9.7% too high. A kilogram is 32.15 troy ounces, so a kilo bar costs 32.15 times the ounce price.

Prices are usually quoted in US dollars. To see the figure in your own currency, convert it with the Currency Converter, which downloads the day’s exchange rates; the amount you enter stays in your browser. Retail prices also include a premium over the market price, and that varies by dealer and product.

The troy pound puzzle

A troy pound has 12 troy ounces, so it weighs 373.2 g — less than an ordinary 16-ounce pound of 453.6 g. A troy ounce is heavier than an ordinary ounce, but a troy pound is lighter than an ordinary pound. Troy pounds are rarely used now; prices are per troy ounce, gram or kilogram.

Purity: karats and fineness

A gold item’s weight is not all gold. Purity is given as karats (out of 24) or fineness (parts per thousand):

• 24 karat ≈ 999 fine: essentially pure
• 22 karat = 91.7% gold (916)
• 18 karat = 75% gold (750)
• 14 karat = 58.3% gold (585 is a common hallmark)
• 9 karat = 37.5% gold (375)

To estimate the gold content, multiply the weight by the purity: a 10 g 18-karat chain contains 7.5 g of gold. The Percentage Calculator handles the sum for any purity. Do not confuse karat (purity) with carat, a unit of mass for gemstones equal to 0.2 g.

Silver, platinum and coins

The same unit applies to silver, platinum and palladium, and bullion coins are usually sold by troy weight: a “1 oz” gold or silver coin contains one troy ounce of the metal. Some coins are made of a gold alloy and weigh more than one troy ounce in total so that the fine gold content is exactly one ounce. The fine weight, often stamped on the coin or stated by the mint, is the figure to value.

Local units

Some markets use their own units — for example the tola in South Asia, about 11.66 g, or the tael in parts of East Asia, whose size varies by region. Check the local definition before converting, and compare prices in grams where you can.

Checking a scale

Jewellers’ scales usually switch between grams, troy ounces and pennyweights (1/20 of a troy ounce, 1.555 g). Check which unit the display is in before writing anything down.

Mistakes to avoid

• Dividing a gold price per ounce by 28.35 instead of 31.10.
• Weighing jewellery on kitchen scales set to ounces and comparing with a troy ounce price.
• Valuing an item by its full weight rather than its gold content.
• Mixing up carats for gemstones with karats for gold.

Checklist

• Price per troy ounce? Divide by 31.1035 for grams.
• Weight in grams and purity known?
• Currency and dealer premium accounted for?