Gold Karat Conversion Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
Gold purity is calculated with (karat ÷ 24) × 100, pure gold being 24 karat. An 18-karat gold item therefore contains 75% pure gold, a fineness of 750 thousandths.
Explanation
The karat measures the proportion of pure gold in a jewelry alloy, on a scale where 24 karat represents 100% pure gold — not to be confused with the carat of weight used for gemstones (a unit of mass worth 200 mg, entirely unrelated). Since pure gold is too soft for most jewelry, it is almost always alloyed with other metals (silver, copper, zinc) for strength: an 18-karat gold item therefore contains 75% pure gold and 25% other metals. In France, the law requires a guarantee hallmark stating the fineness in thousandths rather than in karats: 999 for pure gold, 750 for 18 karat, 585 for 14 karat, and 375 for 9 karat, the latter being the minimum legal fineness for an item to be sold as "gold" in France. This calculator gives the exact theoretical fineness from the karat ÷ 24 calculation; for karats that don't come out round (like 14 or 22 karat), the legal hallmark actually used can differ by a few thousandths from the theoretical result, a nuance detailed in the FAQ below.
Example: an 18-karat gold item
Inputs
Karat: 18.
Calculation
Purity = (18 ÷ 24) × 100 = 75%. Fineness in thousandths = 75 × 10 = 750.
Result
An 18-karat gold item is 75% pure, with a fineness of 750 thousandths.
Frequently asked questions
Why doesn't the calculation for 14 or 22 karat land exactly on the legal hallmark?
The theoretical calculation (karat ÷ 24 × 1000) gives 583 thousandths for 14 karat and 917 for 22 karat, whereas the legal hallmarks actually used in France and many countries are 585 and 916 respectively. This one-to-two thousandths gap comes from legal tolerances and rounding conventions specific to jewelry regulation, not a calculation error: the exact theoretical fineness and the hallmarked legal fineness can differ very slightly for some intermediate karats, unlike 24, 18, 14 (approximate here), and 9 karat, which remain the most reliable reference points.
What is the difference between the karat of gold and the carat of gemstones?
They are two completely different units that only share the same name: the karat of gold measures a proportion of purity out of 24, while the carat of gemstones (diamonds, emeralds) measures a weight, exactly 200 milligrams per carat. A 1-carat diamond weighs 200 mg, independent of any notion of chemical purity.
Why is 24-karat gold rarely used in jewelry?
Because pure gold is a very soft metal, which scratches and deforms easily in daily use. Alloying it with other metals (18 or 14 karat most often) makes it noticeably more wear-resistant, at the cost of reduced purity — a standard industry compromise between durability and precious metal value. To precisely weigh a gold bar or item, see our weight conversion calculator, and to convert its volume measured by displacement, our volume conversion calculator.