Karat, Fineness and Gold Hallmarks Explained
How karat, purity percent and fineness relate, what common hallmark stamps mean, and why stamped numbers are rounded off the exact math.
One purity, three scales
Karat, percentage and fineness are three ways of writing the same thing: how much of an alloy is actually gold. Karat splits gold into 24 parts, so 24 karat is pure and 12 karat is half gold. Percentage expresses the same fraction out of 100, and fineness expresses it in parts per thousand. Because they all describe one ratio, converting between them is a matter of scaling: multiply the karat fraction by 100 for percent or by 1000 for fineness. That is exactly what this calculator does from whichever value you enter.
The math behind the numbers
The core relationship is purity = karat / 24. From there, percent = karat / 24 * 100 and fineness = karat / 24 * 1000. Running it in reverse, karat = percent / 100 * 24 and karat = fineness / 1000 * 24. So 18 karat is 75% and 750 fineness, 22 karat is about 91.6667% and 916.67 fineness, and 10 karat is roughly 41.6667% and 416.67 fineness. Many of these land on repeating decimals, which is why the tool reports karat and percent to four places and fineness to two.
Reading a hallmark stamp
Jewelry is usually stamped with either a karat mark or a fineness mark. A karat mark looks like 14K or 18K, while a fineness mark is a three-digit number such as 585, 750 or 916. Those fineness stamps are deliberately rounded: 585 stands in for 14 karat even though the exact figure is 583.33, and 916 represents 22 karat although the math gives 916.67. When you convert a clean karat value here you see the precise fineness, which helps you understand why a stamp does not always match to the last digit.
Why exact percent still matters
Rounded stamps are fine for identification, but the precise percentage matters when money is involved. Scrap buyers and refiners price by actual gold weight, so a 0.3% gap between the stamped 583 and the true 583.33 fineness scales up on larger lots. Knowing the exact purity also lets you sanity check a dealer quote or compare two pieces stamped in different systems, for instance a 585 European mark against a 14K North American mark. This calculator gives you the unrounded figure so your own valuation math starts from the right number.