Carat (ct) measures gold purity in twenty-fourths. Pure gold is 24 carat; 18ct gold is 18/24 or 75% pure; 9ct — the UK’s most common jewellery standard — is 37.5% pure. The remainder is alloy metals such as copper, silver and zinc, added for hardness and colour. Don’t confuse gold carats with diamond carats, which measure weight rather than purity.
British hallmarks state purity as a fineness number instead: 375 (9ct), 585 (14ct), 750 (18ct), 916 (22ct) and 999 (24ct). Any gold item over one gram sold in the UK must legally carry a hallmark from one of the four UK assay offices (London, Birmingham, Sheffield or Edinburgh), so the stamp is the most reliable guide to what you own. Items below one gram may be exempt, which is why very small pieces can be harder to verify.
For valuation, only the pure gold content counts. A 10-gram 9ct ring contains 3.75 grams of fine gold, so its melt value is 3.75 times the live gram price — roughly a third of what the same weight of 24ct would fetch. This is why two rings of identical weight can have very different values: the carat, not the size, determines the gold content.
Colour is partly a guide but not a reliable one. 9ct gold can look yellow, rose or white depending on the alloy mix, and plated items can mimic higher carats. If there is any doubt about purity, a jeweller or pawnbroker can acid-test an item in seconds. Never assume carat from colour alone when a significant sum is at stake.
When selling scrap, the buyer will test purity themselves regardless of what you believe the hallmark says, so knowing the carat in advance simply helps you spot a fair offer. Our gold gram calculator lets you switch carat and see the live melt value for any weight, so you can value a 9ct ring, an 18ct chain and a 22ct bangle in seconds using the same live price.