Avoirdupois vs troy ounce: which gram value to use
The everyday ounce and the precious-metals ounce weigh different amounts. Here is how they differ and when each gram conversion applies.
Two ounces that are not the same weight
The word ounce hides two distinct units that convert to different gram values. The avoirdupois ounce is the ordinary ounce used for food, parcels and body weight, and it equals exactly 28.349523125 grams. The troy ounce, reserved for gold, silver, platinum and other precious metals, equals about 31.1035 grams, roughly 10 percent heavier. Because the names are identical in speech, mixing them up is one of the most common conversion mistakes.
Where each ounce is used
Almost everything you weigh day to day uses the avoirdupois ounce, including recipe amounts, shipping weights and nutrition labels, which is exactly what this converter is built for. The troy ounce lives in the bullion and jewellery trade, where a gold price is quoted per troy ounce and a coin's mass is stated in troy units. If a figure comes from a mint, an assay office or a metals exchange, assume troy; otherwise assume avoirdupois.
Why the two systems diverged
The split is historical. The troy system, named after the French market town of Troyes, was standardised for weighing coins and precious metals, where small, consistent units mattered. The avoirdupois system, whose name comes from French for goods of weight, grew up around trading bulk commodities. They use different subdivisions too: a troy ounce is one twelfth of a troy pound, while an avoirdupois ounce is one sixteenth of a heavier avoirdupois pound, so the pounds differ as well as the ounces.
Avoiding the costly mix-up
The stakes are highest with precious metals, where a 10 percent unit error translates directly into money. If you weigh a gold item on a normal kitchen scale reading avoirdupois ounces and convert with 28.349523125, you will understate its troy weight and misprice it. When accuracy matters, confirm which ounce your source used before converting, and use a troy-specific factor for metals. For food, mail and general use, the avoirdupois value in this tool is the right one.