Troy Ounce vs Avoirdupois Ounce — Why It Matters When Buying Gold
There are two kinds of "ounce" in common use, and they aren't the same. Confusing them when buying or selling precious metals can cost you real money — about 10% per ounce. This explains what each is, when each is used, and a converter for when you need it.
Troy ⇄ Avoirdupois converter
Both units convert via grams. 1 troy oz = 31.1034768 g · 1 avdp oz = 28.3495231 g.
The numbers
- 1 troy ounce = 31.1034768 grams
- 1 avoirdupois ounce = 28.3495231 grams
- That makes a troy ounce ~9.7% heavier than an avdp ounce
- Conversely, 16 avdp oz = 1 lb but only 14.583 troy oz fit in a pound — making the troy pound (12 troy oz) a different unit again, used historically by apothecaries
When each is used
| Unit | Where you'll see it |
|---|---|
| Troy ounce (ozt) | Precious metals — gold, silver, platinum, palladium spot prices, bullion bars, coin weights, mint pricing |
| Avoirdupois ounce (oz) | Food, post, everyday weight — the ounce on your kitchen scale |
| Gram | Almost everything else, including most modern jewellery weights |
| Pennyweight (dwt) | US scrap-gold trade. 1 dwt = 1.5552 g = 1/20 of a troy ounce |
Why two ounces?
Historical accident, like most weights and measures. The troy system was used in medieval markets at Troyes, France, for precious metals and apothecary goods. The avoirdupois system ("goods of weight") came from 13th-century England for general trade. Both survived, both were standardised in different industries, and neither was retired — so today you have an ounce of gold that doesn't weigh the same as an ounce of butter.
How this trips people up
- Weighing on a kitchen scale. Your kitchen scale shows avdp ounces. If a jeweller quotes the spot price "per ounce" and you weigh your scrap on an avdp scale, you'll under-count by about 10%. Always weigh in grams, then convert.
- Importing from the US. American scrap-gold dealers often quote in pennyweights (dwt) not grams. 1 dwt = 1.5552 g. The price per pennyweight will look smaller than per gram — that's the unit, not a worse offer.
- Bullion bar weights. A "1 oz" gold bar is 1 troy ounce = 31.1g. A "1 kilo" bar is 32.1507 troy ounces.
- Mixed kitchen-scale and price-per-ozt. The combination is exactly where most home-stackers lose money. Switch to grams everywhere.
Useful conversions to memorise
- 1 troy oz = 31.1 g (close enough for back-of-envelope)
- 1 kg = 32.15 troy oz
- 1 troy oz = 1.0971 avdp oz
- 1 sovereign = 7.988 g = 0.2568 troy oz
- 1 Britannia 1 oz coin = 31.1 g = 1 troy oz exactly
The pound problem
If "ounce" is two units, "pound" is even worse:
- The avoirdupois pound (lb) = 16 avdp oz = 453.59 g — your everyday pound
- The troy pound = 12 troy oz = 373.24 g — historical apothecary measure, almost never used today
You will rarely encounter the troy pound. If anyone quotes you "gold per pound" without specifying, ask. (And probably walk away — reputable dealers quote per gram, per troy ounce, or per kilo.)