Avoirdupois Ounce
Also: av. oz, commercial ounce, avoirdupois ounce
The avoirdupois ounce (av. oz) is the mass unit used in everyday Anglo-American commerce, equal to exactly 28.349523125 grams — noticeably different from the troy ounce of 31.1035 grams used in the precious-metals trade.
The avoirdupois ounce (abbreviated av. oz or oz av.) is the everyday mass unit of the English-speaking world for food, personal care and general merchandise. It descends from the medieval Franco-English trading system (avoir de pois — "goods of weight") and has been uniformly defined since the British Weights and Measures Act of 1824. In the unit converter you can convert the avoirdupois ounce comfortably into grams, troy ounces or other measures.
Avoirdupois ounce versus troy ounce
Anyone buying or selling precious metals needs to be alert to the risk of confusing it with the troy ounce (ozt). The troy ounce, from the troy weight system, is the globally binding unit for gold, silver, platinum and palladium — and it is roughly 10 % heavier than the avoirdupois ounce:
| Unit | Grams | Grains |
|---|---|---|
| Troy ounce (ozt) | 31.1035 g | 480 gr |
| Avoirdupois ounce (av. oz) | 28.3495 g | 437.5 gr |
| Difference | −2.754 g | −42.5 gr |
Anyone weighing silver jewellery, scrap silver or broken gold in avoirdupois ounces and then valuing it against the silver price will arrive at too low a material value — the correct factor is:
troy ounces = avoirdupois ounces × 0.911458
Conversion into common units
- 1 av. oz = 28.349523125 g (exact, defined by statute)
- 1 av. oz = 0.9115 ozt (troy ounces)
- 1 av. oz = 437.5 grains
- 1 kilogram = 35.274 av. oz
Where the avoirdupois ounce turns up in precious metals
In the professional precious-metals trade only the troy ounce is quoted — as it is on the gold price page and on COMEX. Private individuals still meet the avoirdupois ounce in the following situations:
- Jewellery buy-back: jewellers in the US and Britain sometimes weigh gold jewellery in av. oz.
- Imported scales: budget kitchen scales from outside the EU often show "oz" with no qualifier — the avoirdupois ounce is meant.
- Online marketplaces: listings from the US quote weights in oz av.; when using the melt-value calculator, be sure to select the correct unit.
In brief
The avoirdupois ounce is a shade under 9 % lighter than the troy ounce and is not used in the precious-metals trade. Anyone converting everyday weights into precious-metal values must always apply the factor 0.9115, or reach straight for the unit converter, to avoid calculation errors.