Avoirdupois Ounce
Also: av. oz, commercial ounce, avoirdupois ounce
The avoirdupois ounce (av. oz) is the mass unit commonly used in everyday Anglo-American life, equal to exactly 28.349523125 grams, and therefore differs markedly from the troy ounce used in the precious metals trade at 31.1035 grams.
The avoirdupois ounce (abbreviation: av. oz or oz av.) is the mass unit widely used in everyday English-speaking life for food, personal care and general merchandise. It stems from the medieval French-English trading system (avoir de pois – "goods of weight") and has been uniformly defined since the British Weights and Measures Act of 1824. With the unit converter the avoirdupois ounce can be conveniently converted into grams, troy ounces or other measures.
Avoirdupois ounce vs. troy ounce
Anyone buying or selling precious metals must be aware of the risk of confusion with the troy ounce (ozt). The troy ounce from the troy weight system is the globally binding unit for gold, silver, platinum and palladium – it is around 10% heavier than the avoirdupois ounce:
| Unit | Grams | Grain |
|---|---|---|
| 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 who weighs silver jewellery, scrap silver or broken gold in avoirdupois ounces and calculates the value via the silver price obtains 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, legally defined)
- 1 av. oz = 0.9115 ozt (troy ounces)
- 1 av. oz = 437.5 grain
- 1 kilogram = 35.274 av. oz
Where the avoirdupois ounce appears in the precious metals field
In the professional precious metals trade only the troy ounce is quoted – as with the gold price and on COMEX. The avoirdupois ounce nevertheless meets private individuals in the following situations:
- Jewellery purchase: Jewellers in the USA and the UK sometimes weigh gold jewellery in av. oz.
- Scale imports: Cheap kitchen scales from outside the EU often display "oz" without a qualifier – meaning the avoirdupois ounce.
- Online marketplaces: Offers from the USA list weights in oz av.; in the melt value calculator always choose the correct unit.
In brief
The avoirdupois ounce is almost 9% lighter than the troy ounce and is not in common use in the precious metals trade. Anyone wishing to convert everyday weight figures into precious metal values must always apply the factor 0.9115 or use the unit converter directly to avoid calculation errors.