There are two completely different “ounces” in everyday use, and confusing them can cost you 10% of your money. UK bullion buyers run into this every time they weigh scrap on a kitchen scale.

The numbers

  • 1 troy ounce (oz t) = 31.1034768 grams
  • 1 avoirdupois ounce (oz) = 28.349523125 grams
  • A troy ounce is ~10% heavier than an avoirdupois ounce.

If you weigh “1 ounce” of gold on a kitchen scale you’ll get 28.35g — but the price quoted “per ounce” on this site, on every bullion dealer’s site and on the LBMA fix, is per troy ounce, i.e. per 31.10g.

Why two ounces exist

Both date to medieval Europe and were used in parallel for centuries:

  • Avoirdupois (from the Old French aveir de peis, “goods of weight”) — the everyday weight system for food, livestock and general goods. 16 ounces to the pound. Still used in the US and informally in the UK for things like baby weights, recipes, postage.
  • Troy — used specifically for precious metals and gemstones since at least the 15th century, named after the French city of Troyes which was a major trading hub. 12 troy ounces to one troy pound.

Britain officially abandoned the troy pound in 1878, but the troy ounce survived because it’s the global standard for precious-metals trading. Every major exchange — LBMA, COMEX, Shanghai Gold Exchange — quotes per troy ounce.

Live gold price per troy ounce →

The pound trap

Be careful:

  • 1 avoirdupois pound (the supermarket pound) = 16 oz × 28.35g = 453.59g.
  • 1 troy pound = 12 oz t × 31.10g = 373.24g.

So a “pound of gold” in old reckoning is 80g lighter than a pound of feathers. The phrase “which weighs more, a pound of gold or a pound of feathers?” is a real-life riddle: a pound of feathers (453g, avoirdupois) weighs more than a pound of gold (373g, troy) because they use different pound systems.

Practical scrap weighing

If you bring scrap gold or silver to a UK dealer:

  1. The dealer weighs it on a gram scale (always).
  2. They convert grams to troy ounces internally if they think in those terms.
  3. They quote “we pay £X per gram for 18ct” — gram-based pricing avoids the troy/avp confusion entirely.

If a buyer ever quotes you “per ounce” without specifying which, always assume troy for precious metals (it’s standard) but ask explicitly. A 10% difference matters.

Worked example

You have a 50g piece of 22ct (916) gold:

  • Pure gold content: 50 × 0.916 = 45.8g.
  • In troy ounces: 45.8 / 31.10 = 1.473 oz t.
  • In avoirdupois ounces: 45.8 / 28.35 = 1.616 oz.

At a £85/g spot price:

  • Spot value: 45.8 × £85 = £3,893.
  • Expressed per troy ounce: 1.473 × £2,644/oz t = £3,893.
  • Expressed per avoirdupois ounce: 1.616 × £2,409/oz = £3,893.

The pound figure is the same — only the unit changes.

Bullion value calculator — handles both ounce types →

The other units you’ll meet

Bullion has more than two weight systems in active use:

  • Gram (g) — universal scientific standard. Every UK dealer prices scrap per gram.
  • Kilogram (kg) — 1,000g. Wholesale and institutional sizes. A “good delivery” gold bar is ~12.5kg.
  • Troy ounce (oz t) — the bullion-market standard. 31.10g.
  • Avoirdupois ounce (oz) — the everyday ounce. 28.35g. Almost never used for bullion pricing.
  • Pennyweight (dwt) — 1.555g. US jewellers still use this. (Read more.)
  • Grain (gr) — 0.0648g. Obscure, used for very small amounts. (Read more.)
  • Tola — 11.66g. Indian / South Asian standard. (Read more.)
  • Baht — 15.24g. Thai gold standard.
  • Mace — 3.78g. Chinese gold standard.

All of them are supported by the bullion calculator — pick your unit and let it convert.

Rule of thumb

In bullion, “ounce” always means troy ounce — 31.10 grams.

If anyone quotes a precious-metals price “per ounce” without qualifying it, they almost certainly mean troy. The exception is some jewellery contexts in the US where pennyweights are still used — but even there, scrap is increasingly priced per gram.