18-karat gold has 18 parts of gold for every 24 parts of alloy, which is 75% of its mass. That purity is the fraction 18/24, which can be simplified. A 4 g piece of 18-karat gold contains 3 g of pure gold, and the rest is other metals. The 24 is the whole: pure gold, at 24 karats, would be 24/24.
This article shows how to read any karat number as a fraction of 24, convert it to a percentage and to fineness (parts per thousand), and find the grams of gold in a piece. To simplify the fraction with other numbers, use the simplify fractions calculator.
What a gold karat measures
In jewelry, one karat is 1/24 of the total mass of the alloy. So the number next to “K” or “kt” tells you how many of those 24 parts are gold. According to Wikipedia, 18 karat is 18/24, or 75% gold.
In American English, “karat” (with a K) is the usual spelling for gold purity and “carat” (with a C) is the unit for gemstones, covered below.
Purity is a proportion, not an amount. Two 18-karat rings, one of 6 g and one of 20 g, have the same proportion of gold, but they hold 4.5 g and 15 g of precious metal. The fraction describes the mix, and the mass tells you how much material there is.
From karats to a fraction, a percentage and fineness
The method is the same for every value. First write the karats over 24 and simplify with the greatest common divisor. Since GCD(18, 24) = 6, the fraction reduces to 3/4. Then multiply by 100 for the percentage, 75, and by 1000 for the fineness, 750. The fraction to percent calculator does that last step for you.
| Karats | Fraction | Percentage | Fineness |
|---|---|---|---|
| 24 | 1 | 100% | 1000 |
| 22 | 11/12 | ≈ 91.67% | ≈ 917 |
| 18 | 3/4 | 75% | 750 |
| 14 | 7/12 | ≈ 58.33% | ≈ 583 (stamp 585) |
| 10 | 5/12 | ≈ 41.67% | ≈ 417 |
| 9 | 3/8 | 37.5% | 375 |
Among the rows with a proper fraction, only 18 and 9 karats give terminating decimals, because their denominators, 4 and 8, have only the factor 2. The others have denominator 12, which has the factor 3, so their decimals repeat. Values marked “≈” are rounded. With 11/12 = 91.666…%, 917 thousandths is above the exact value by 1/3 of a thousandth; for 14 and 10 karats (583.33… ‰ and 416.67… ‰, rounded to 583 and 417) the error is also 1/3 of a thousandth. For the percentages, rounding to two decimals leaves an error under 0.005 percentage points. The guide to exact and repeating decimals explains why.
How many grams of pure gold are in a piece
Multiply the mass by the purity fraction. It is a fraction of a number, with grams as the unit.
Worked example: a 10 g chain of 18-karat gold
A chain weighs 10 g and is stamped 18 karats. How many grams of pure gold does it contain?
- Write the purity as a fraction. 18 karats reduces to 3/4.
- Multiply by the mass. 10 × 3/4 = 15/2.
- Convert to a decimal. 15/2 = 7.5 g of gold.
- Find the rest. 10 − 7.5 = 2.5 g of other metals.
Result: 7.5 g of pure gold.
Check: 7.5 is 75% of 10, and 7.5 + 2.5 gives back the 10 g of the chain.
When the mass is not a multiple of the denominator, the decimal does not terminate. With 8 g of 14-karat gold you get 8 × 7/12 = 14/3 g, which is 4.666… g, or 4.67 g with an error under 0.004 g. Keep the fraction while you are still calculating and round only at the end.
Why the 14-karat stamp reads 585
The exact fineness of 14 karats is 7/12, which in thousandths is 583.333… and never ends. Wikipedia lists 14 karat as 585 thousandths, 18 karat as 750 and 9 karat as 375. Reading 585 as a conventional value close to the exact one is a deduction of this page, not of that source. The difference from the exact value is 585/1000 − 7/12 = 1/600, a little under two thousandths of the piece. That is why you see 585 engraved even though the fraction calculation gives 583.3. The grams of a piece with that stamp can be calculated with 7/12 or with 585/1000, depending on the case, and it helps to say which one was used.
Don’t confuse the gold karat with the gemstone carat
For gemstones, the carat is a unit of mass equal to 200 mg, according to Wikipedia. It does not measure purity, and a 1-carat diamond weighs 0.2 g. The two share a name but measure different things. If you see “carats” next to a stone, it refers to weight; next to a metal, to the proportion of gold.
What the karat number doesn’t tell you
The karat number gives the proportion of gold, but not which other metals are in the alloy, how much the piece weighs, or what it is worth. The price also depends on the day’s gold price, which this page does not cover. If the stamp is worn, the engraved number is a label and not a measurement. For more part-and-whole problems with real quantities, see the applications of fractions and the guide to fractions in everyday life.
Source: Wikipedia, articles Carat (purity) and Carat (mass), on the gold karat and the gemstone carat (accessed October 16, 2026).