The troy ounce takes its name from the medieval trade fairs of Troyes in France, where merchants needed a weight standard both sides could trust. Eight centuries later it is still the unit in which the entire professional bullion market operates. One troy ounce equals exactly 31.1034768 grams — about 10% heavier than the kitchen (avoirdupois) ounce of 28.35 grams, a distinction that has caught out more than one hasty buyer.
The three conversions worth memorising
One troy ounce is 31.1035 grams. One kilogram is 32.1507 troy ounces. And the price per gram is the ounce price divided by 31.1035 — so gold at €3,500 per ounce is about €112.53 per gram. International markets quote per ounce, while European retail products are labelled in grams; being fluent in both directions lets you read any price list at a glance. Our calculators do the arithmetic at the current reference price, for any weight and purity.