Troy ounces and gold purity

Every XAUUSD quote prices one troy ounce of pure gold. Both halves of that sentence carry precision worth knowing: the troy ounce is heavier than the everyday ounce, and purity standards decide what counts as pure. None of this changes how you trade, but it anchors what the number on the screen physically means.

The troy ounce

A troy ounce is about 31.103 grams, roughly ten percent heavier than the avoirdupois ounce used for groceries, at about 28.35 grams. Precious metals kept the medieval troy system while everything else moved on. Twelve troy ounces make a troy pound, though nobody quotes gold that way; grams and kilograms dominate outside the dollar quote.

Karats and fineness

Karats measure purity in twenty-fourths: 24k is pure gold, 18k is eighteen parts gold to six parts other metal. Bullion uses fineness instead: parts per thousand. Investment-grade bars are typically 999.9 fine, market shorthand "four nines". The spot price refers to this pure metal; jewellery prices diverge because they price craftsmanship and lower purity.

From ounce price to anything else

Divide the spot price by 31.103 for the per-gram price; multiply the gram price by a thousand for the kilo bar price. A quote of 4,400 per ounce is roughly 141 per gram. Handy when comparing the chart with physical-market prices quoted per gram or per tola in much of the world.

Why contracts standardised

Futures settle in 100-troy-ounce units and good-delivery bars run around 400 ounces because standardisation is what makes gold fungible: any conforming bar settles any contract. The chart's single clean number exists because the physical world agreed on units first.

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