About the Gold Karat Purity
This calculator converts a gold item's karat rating, out of 24, into its purity percentage and millesimal fineness, the three-digit number stamped on many gold hallmarks. It is useful for jewelry shoppers, appraisers, and jewelry makers who need to translate between the karat system common in the US and the fineness numbers used on hallmark stamps internationally.
How It Works
You enter a karat value out of 24, where 24 represents pure gold. The calculator divides that value by 24 and multiplies by 100 to get the purity percentage, then multiplies the percentage by 10 and rounds to the nearest whole number to get the millesimal fineness.
Formula & Methodology
Millesimal fineness expresses purity out of 1000 rather than out of 24, so it is simply the percentage figure with the decimal point shifted one place and rounded to a whole number. By hand: divide the karat number by 24, convert that fraction to a percentage, then multiply by 10 to land on the fineness stamp you would expect to see on the item, such as 750 for 18K or 585 for 14K in markets that round slightly differently.
Examples
Standard 18K jewelry
An 18K item computes to exactly 75.00% purity and a millesimal fineness of 750, matching the '750' hallmark commonly stamped on 18K gold pieces.
Common 14K jewelry
A 14K item computes to 58.33% purity and a fineness of 583, close to the '585' fineness mark used in many countries for 14K gold alloys.
Advantages
- Instantly matches a karat number to both its percentage purity and its hallmark fineness figure.
- Helps compare jewelry described with different labeling systems, such as US karat versus European fineness stamps.
- Removes manual division errors when appraising, buying, or casting gold alloys.
Common Mistakes
- Confusing karat, which measures gold purity out of 24, with carat, which measures gemstone weight in units of 0.2 grams.
- Assuming 24K gold is the most practical jewelry choice, when its softness makes it prone to scratching and bending compared to 18K or 14K alloys.
- Misreading a fineness hallmark like 585 as a karat number rather than as a purity fraction out of 1000.
Edge Cases to Watch For
- Karat values above 24 are not physically meaningful, since pure gold is capped at 24K, but the calculator does not block such an entry and will still compute a result above 100% purity.
- Fractional karat values, such as 14.5K used in some regional markets, are accepted since the input simply accepts any number.
- Fineness is rounded to the nearest whole number, so karat values close together can round to the same fineness figure.
Common Use Cases
- Jewelry shoppers comparing 14K, 18K, and 22K pieces before purchase.
- Appraisers and pawnbrokers cross-checking a hallmark's fineness number against a stated karat.
- Jewelry makers calculating the gold content needed when alloying metal for casting.