Avoirdupois Ounce
Also: av. oz, Trade ounce, Avoirdupois ounce
The avoirdupois ounce (av. oz) is the mass unit commonly used in everyday Anglo-American commerce, equalling exactly 28.349523125 grams, and differs significantly from the troy ounce used in precious metal trading, which weighs 31.1035 grams.
The avoirdupois ounce (abbreviation: av. oz or oz av.) is the mass unit widely used in English-speaking everyday life for food, personal care products, and general trade goods. It originates 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. The avoirdupois ounce can be conveniently converted to grams, troy ounces, or other measures using the unit converter.
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 approximately 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 who weighs silver jewellery, scrap silver, or broken gold in avoirdupois ounces and calculates the value using the silver price will obtain a material value that is too low – the correct conversion factor is:
Troy ounces = Avoirdupois ounces × 0.911458
Conversion to Common Units
- 1 av. oz = 28.349523125 g (exact, legally defined)
- 1 av. oz = 0.9115 ozt (troy ounces)
- 1 av. oz = 437.5 grains
- 1 kilogram = 35.274 av. oz
Where the Avoirdupois Ounce Appears in Precious Metal Contexts
In professional precious metal trading, only the troy ounce is quoted – as is the case on the gold price and at COMEX. Private individuals nevertheless encounter the avoirdupois ounce in the following situations:
- Jewellery purchase: Jewellers in the USA and UK sometimes weigh gold jewellery in av. oz.
- Imported scales: Inexpensive kitchen scales from outside the EU frequently display "oz" without any qualifier – meaning the avoirdupois ounce.
- Online marketplaces: Listings from the USA show weights in oz av.; always select the correct unit when using the melt value calculator.
In Brief
The avoirdupois ounce is approximately 9% lighter than the troy ounce and is not used in precious metal trading. Anyone wishing to convert everyday weight specifications into precious metal values must always apply the factor 0.9115 or use the unit converter directly to avoid calculation errors.