Gold Purity (Karat to %) Calculator
Gold purity is described in two main ways, and this calculator converts between them. The karat system measures purity in twenty-fourths, so 24 karat is pure gold and a lower karat is gold blended with other metals to add strength and lower cost. The percentage and fineness systems express the same purity differently: the percentage is the share of gold by weight, and the fineness is that share written in parts per thousand. This gold purity calculator takes a karat value and returns both the gold percentage and the fineness, so you can read a hallmark, compare pieces, or value scrap on a consistent basis. For example, 18 karat gold is 18 parts gold out of 24, which is 75 percent pure, or 750 fineness, often stamped simply as 750. Every figure here is computed deterministically from the karat you enter, so the same input always returns the same purity. Enter a karat value below to convert a hallmark, check a jeweler's quote, or compare two pieces on the same scale, with a worked example that reconciles exactly to the calculator default so you can follow each step. Purity describes the metal content only, not the market value, which also depends on weight and the gold spot price.
Gold percentage equals the karat divided by 24, times 100, and fineness is parts per thousand. 18 karat gold is 75.00% pure, or 750 fineness.
Gold purity formula
gold percentage = (karat / 24) x 100
fineness = (karat / 24) x 1,000
24 karat = pure gold; karat is parts of gold out of 24
Karat measures gold content in twenty-fourths. Dividing the karat by 24 gives the gold fraction, which becomes a percentage when multiplied by 100 and a fineness when multiplied by 1,000.
Worked example
Take 18 karat gold and convert it to a percentage and fineness.
- Gold fraction: 18 / 24 = 0.75
- Gold percentage: 0.75 x 100 = 75.00%
- Fineness: 0.75 x 1,000 = 750 parts per thousand
18 karat gold is 75.00 percent pure, or 750 fineness, which matches the calculator's default input exactly.
Gold Purity (Karat to %) Calculator: frequently asked questions
What does 24 karat mean?
24 karat is pure gold, with no other metals added. The karat number is the parts of gold out of 24, so 18 karat is 18 parts gold and 6 parts other metals, and 12 karat is exactly half gold by weight.
What is fineness?
Fineness expresses purity in parts per thousand. It is the gold fraction multiplied by 1,000, so 75 percent pure gold has a fineness of 750. Hallmarks often stamp the fineness directly, such as 750 for 18 karat or 916 for 22 karat.
Why is pure gold not used for jewelry?
Pure 24 karat gold is soft and scratches and bends easily. Alloying it with metals such as copper, silver, or palladium increases hardness and durability, and can also change the color, which is why 18 and 14 karat are common for everyday jewelry.
Does purity tell me the value?
Not on its own. Purity is the gold content as a fraction, but the dollar value of a piece also depends on its weight and the current gold spot price. Use a gold scrap value calculator to combine purity, weight, and price into a value.
What is the most common jewelry karat?
It varies by region, but 14 and 18 karat are widely used. 14 karat (about 58.3 percent gold) is harder and cheaper, while 18 karat (75 percent gold) is richer in color and value. 22 and 24 karat are common in some markets for investment and bridal pieces.
Official sources
- Coin metal content and precious-metal specifications: US Mint. As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.