Gold purity converter
Convert karats to percentage and to the fineness number stamped inside jewellery: 750 means 18 karat.
That's about the same as 18 karat gold. A 10 g piece at that purity holds about 7.5 g of pure gold.
What it means
How pure is that?
Karat measures purity, not weight. Carat with a c is a gemstone weight of 200 mg, and a different thing entirely.
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Three things worth knowing
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Divide by 24
18 ÷ 24 = 0.75, so 18 karat is 75% gold, stamped 750. 14 karat is 58.3%, hallmarked 585.
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Carob seeds
Both karat and carat come from the carob seed, once used as a tiny standard weight. A Roman gold coin weighed 24 seeds, which is where 24 karat comes from.
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Karat is not carat
A 2-carat diamond weighs 400 mg. A 22-karat ring is 91.7% gold. Same sound, unrelated measures.
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Background
About gold purity
Pure gold is 24 karat, so karats are simply 24ths: 18 karat is 18 parts gold and 6 parts other metals, or 75%. Jewellery is stamped with millesimal fineness instead: 750, 585 or 375.