Carats vs karats — one is mass, one is purity
Carats measure diamond mass. Karats measure gold purity out of 24. The spelling differs in the US and merges everywhere else.
Both words come from the Greek 'keration', the carob seed used historically as a small counterweight. The two modern meanings diverged in medieval trade — one for gem mass, one for gold purity — and English preserved both. Confusingly, most of the world spells both 'carat'.
The exact factors
Diamond carat (ct) = 200 mg = 0.2 g exactly > Gold karat (US) / carat (UK): fraction of 24 > 24k = 99.9% pure gold (999 fineness) > 22k = 91.67% > 18k = 75.0% > 14k = 58.33% > 10k = 41.67% (US legal minimum for 'gold') > 9k = 37.5% (UK legal minimum)
Diamond size perception
A 1 ct round diamond is about 6.5 mm across. A 2 ct is 8.2 mm — only 26% wider despite doubling the mass, because volume scales with the cube of diameter. This is why per-carat pricing is not linear: a 2 ct diamond is typically 3–4× the price of a 1 ct of the same quality.
Gold alloys and their colours
Rose gold: copper alloy. White gold: nickel or palladium alloy, often rhodium-plated. Green gold: silver alloy. All can be sold at any karat mark, because karat rates only the gold content — the balance can be any base metal, subject to hallmarking rules.
Worked example — a 14k gold ring, 5 grams
5 g × 58.33% = 2.917 g of pure gold = 0.0938 troy oz. At a gold spot price of $2,300/troy oz, the gold content is worth $215.70. Retail price will include workmanship, retailer margin, and any diamond content — often 3–10× the raw gold value.
The five-second recap
Carat (ct) = 0.2 g of diamond. Karat (k) = /24 of gold purity. The words look alike; the units are unrelated.
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FAQ
Why 200 mg for a carat?+
The 'metric carat' was standardised in 1907. Before that, carats varied from 188 to 213 mg regionally.
Is 24k gold pure?+
Practically yes — 99.9%. True 100% pure gold does not exist commercially; it is too soft to work and always contains trace impurities.
Do platinum rings use karats?+
No — platinum uses parts-per-thousand marks (Pt950 = 95% pure). Karat is a gold-only convention.
This article was written by The Turbo Unit Converter engineering desk (Jewellery & gemmology) and last reviewed on 2026-06-28 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.