Troy Ounce vs Ounce
A troy ounce is 31.1034768 grams, roughly 9.7 percent heavier than the avoirdupois ounce of 28.349523125 grams used for everything else.
By Piero Nanni · Last checked
Two ounces, and two pounds behind them
The troy system survives only for precious metals, gemstones and some pharmaceutical contexts. Everything else in the English-speaking world uses the avoirdupois system, which is where the familiar 16-ounce pound comes from.
The quirk that catches people out is that a troy pound is lighter than an avoirdupois pound despite having heavier ounces, because it contains only 12 of them rather than 16.
| Measure | Troy | Avoirdupois |
|---|---|---|
| Ounce | 31.1034768 g | 28.349523125 g |
| Ounces per pound | 12 | 16 |
| Pound | 373.24 g | 453.59 g |
| Used for | Gold, silver, platinum, gemstones | Food, freight, body weight, everything else |
What it means for a bullion price
A quoted gold price is per troy ounce. Dividing that price by 28.35 to get a per-gram figure produces a number about 9.7 percent too high, because the correct divisor is 31.1035.
Working the other way is where the error costs money. A one kilogram gold bar is 32.15 troy ounces, not the 35.27 avoirdupois ounces a general-purpose converter would give. Valuing it with the wrong ounce overstates it by roughly a tenth.
Where the name comes from
The system is generally traced to Troyes in the Champagne region of France, a major medieval trade fair town, though the derivation is not certain. It reached England by the fifteenth century and became the standard for coinage and precious metal.
Britain abolished troy weight for general trade in 1878 but kept the troy ounce, which is why it is the one troy unit still in daily commercial use. The troy pound, grain and pennyweight survive mainly in specialist trades.
Frequently Asked Questions
How many grams in a troy ounce?
31.1034768 grams, against 28.3495 grams for a standard ounce.
How many troy ounces in a kilogram of gold?
32.1507 troy ounces.
Is a troy pound heavier than a normal pound?
No. A troy pound is 373.24 g against 453.59 g, because it holds 12 ounces rather than 16.
Why is gold priced in troy ounces?
The troy system became the standard for coinage and precious metal in medieval Europe and was retained for bullion when other troy units were dropped.