The Troy Ounce: Why Gold and Silver Use a Different Ounce

The everyday ounce in the US and UK is the avoirdupois ounce. Sixteen of them make an avoirdupois pound. The troy ounce belongs to a separate, older system that survived for precious metals and, historically, for medicines. Both systems are built from the same smallest unit, the grain, which is defined as exactly 64.79891 milligrams. The difference lies in how many grains each ounce contains.

Here is the part that trips almost everyone up. A troy ounce is heavier than an ordinary ounce, but a troy pound is lighter than an ordinary pound. That is because a troy pound contains only 12 troy ounces, while an avoirdupois pound contains 16 avoirdupois ounces. The result: a troy pound is about 373 g and a normal pound about 454 g.

This is also the answer to the old riddle “Which is heavier, a pound of feathers or a pound of gold?” If the feathers are weighed in avoirdupois pounds and the gold in troy pounds, the feathers are heavier. Ask instead about an ounce of each and the gold wins. In practice, troy pounds are rarely used today; the market works almost entirely in troy ounces, grams and kilograms.

The name is generally traced to Troyes, a town in the Champagne region of France that hosted some of the great trade fairs of medieval Europe. Merchants from across the continent met there, and a standard set of weights for valuable goods became associated with the town. The exact path from Troyes to the English troy system is not fully documented, but by the 15th century troy weight was the official English standard for gold and silver, and in 1527 it replaced the older Tower pound for coinage.

When the UK abolished most troy units in the Weights and Measures Act of 1878, it kept the troy ounce for precious metals. That carve-out is why the unit is still with us: the world’s bullion markets, including the London market, quote gold and silver prices in US dollars per troy ounce.

The key ratios are worth memorising if you deal with precious metals:

Kitchen scale check: a 1 troy ounce silver round should read about 1.10 oz on a kitchen scale set to ounces, or about 31.1 g. If it reads exactly 1.00 oz (28.35 g), it contains roughly 9% less metal than advertised.

Valuing a kilobar: a 1 kg gold bar contains 32.1507 troy ounces. If gold is quoted at a given price per ounce, multiply that price by 32.15 — not by 35.27, which is the number of ordinary ounces in a kilogram. Using the wrong factor overstates the value by nearly 10%.

Scrap jewellery: a chain weighing 15 g contains 15 ÷ 31.1035 ≈ 0.482 troy ounces of metal in total. Its gold content depends on its purity (see below), so an 18-karat chain contains about 0.482 × 0.75 ≈ 0.362 troy ounces of pure gold.

A coin described as “1 oz” is usually telling you how much pure metal it contains, not its total weight. Many bullion coins are alloys. The American Gold Eagle, for example, is 22-karat gold alloyed with silver and copper for durability; the one-ounce coin contains one troy ounce of gold but weighs about 33.93 g in total. Coins such as the Canadian Gold Maple Leaf are .9999 fine, so their total weight is very close to 31.10 g.

Purity is expressed either in karats (parts out of 24) or as fineness (parts per thousand).

Outside the London and New York markets you will encounter regional units. In India and neighbouring countries gold is often sold by the tola, about 11.66 g. In Hong Kong and parts of Asia the tael is used, though its exact value varies by region (the Hong Kong tael is about 37.43 g). Large wholesale transactions use kilograms or “good delivery” bars of roughly 400 troy ounces (about 12.4 kg). When in doubt, convert everything to grams — it is the one unit that means the same thing everywhere.