Karat describes purity
Gold karat expresses gold as parts of a total of 24. The nominal arithmetic ratios are 87.5% for 21K, 75% for 18K and approximately 91.67% for 22K. Products described as 24K are high-purity gold, and may state a more precise fineness such as 999 or 999.9 parts per thousand.
Compare the fine-gold content
A 10-gram 18K item has a nominal fine-gold equivalent of 10 × 18 ÷ 24, or 7.5 grams. At the same total weight, 21K gives 8.75 grams. This calculation compares metal content; it does not test a particular item or value its design and stones.
A gram of 18K and a gram of 24K therefore need a purity adjustment before their metal values can be compared. Keep the currency and quote time consistent as well. A calculator can apply the same ratio to the weight you enter.
Read the item's actual description
Purity may be expressed in karats or parts per thousand. A 750 mark represents 75% gold. Some common commercial marks differ slightly from a theoretical karat fraction: 585 is common for 14K, for example. Use documented actual fineness when a more precise estimate is needed.
Colour alone does not establish karat. Ask for the metal description, net metal weight and hallmark, and separate stones and making charges when comparing finished jewellery.