A troy ounce is the precious-metals weight unit equal to 31.1034768 grams, and it is the unit in which the gold price is quoted worldwide. It is heavier than the everyday avoirdupois ounce of 28.3495 grams, so converting a dealer gram price to an ounce price with the wrong figure overstates or understates the premium by about ten percent.
One standard XAUUSD contract is 100 troy ounces, roughly 3.11 kilograms of gold, which is the fact behind the sizing rule that a one dollar move in gold is worth approximately one hundred dollars per lot. Mining costs are reported in dollars per troy ounce and central bank reserves in tonnes, where one tonne equals 32,150.7 troy ounces.