What Is Gold Karat? Purity, Fineness and Carat Weight
Gold karat measures purity on a 24-part scale. Learn how to read the fraction and how it differs from a gemstone's carat weight.
GoldNepal Content Team · Published
Gold karat measures purity on a 24-part scale. An 18K alloy, for example, has a theoretical gold fraction of 18 ÷ 24, or 75%. Karat is not a measure of an item's weight, and it does not by itself verify how a particular piece was made or tested.
Karat is not carat weight
In jewellery, karat (often written K) describes the gold proportion in an alloy. Carat (ct) measures a gemstone's weight. The Gemological Institute of America defines one metric carat as 200 milligrams, or 0.2 grams. A one-carat diamond therefore tells you its weight, not the gold purity of the setting.
How the gold-purity calculation works
Divide the karat number by 24 to get its theoretical gold fraction. For 18K, that is 18 ÷ 24 = 0.75, or 75% by calculation. Fineness expresses precious-metal content in parts per thousand, so the same theoretical fraction is 750 parts per thousand. Actual marking and tolerance conventions vary by market; the arithmetic does not establish that an individual item has been independently assayed.
The World Gold Council explains that lower-karat gold contains a smaller gold fraction and that alloy composition can affect colour. The karat number alone does not give the full alloy recipe, construction, durability, maker, or final price.
What to check on an individual item
Ask what the stated purity refers to and whether the available documentation or testing applies to that piece. If a design includes gemstones, keep the stone's carat weight separate from the metal's weight and gold-purity rating. This guide explains the terms; it does not assess a specific item or interpret local marking law.
For the specific 24K-versus-22K arithmetic, read 24K vs 22K gold. For the specific 18K-versus-14K comparison, see 18K vs 14K gold. GoldNepal's weight converter converts weight units; it does not test purity or calculate gemstone grade.