Gold karats, grams and troy ounces: avoid double-counting purity
Estimate the metal value of 18K, 21K, 22K and 24K gold using the correct weight unit and a single purity adjustment.
Match the price and weight units
Precious-metal quotations commonly use the troy ounce: exactly 31.1034768 grams. An ordinary ounce is 28.349523125 grams. Confusing them changes a valuation materially. The NIST precious-metals conversion reference documents the troy conversion. Do not use an ordinary ounce conversion for a troy-ounce gold quote.
Convert karat to a theoretical gold fraction
Karat expresses parts of gold out of 24. Thus 18K is 18 ÷ 24 = 75%, 21K is 87.5%, and 22K is about 91.67%. These fractions are a calculation model, not a substitute for an assay or a guarantee about an item. A hallmark, invoice and independent testing may be needed to establish actual fineness.
Apply purity once, not twice
Assume pure gold costs 80 currency units per gram. Ten grams of 21K gold has a theoretical metal value of 10 × 80 × 21 ÷ 24 = 700. Alternatively, the corresponding 21K gram price is 70, giving 10 × 70 = 700. If your quote already says “21K per gram,” do not multiply by 21/24 again. Use the selected-karat setting in the gold value calculator.
Separate metal value from a shop offer
Jewellery prices can include workmanship, stones, tax and a retail premium. Resale offers may deduct refining costs and dealer margins. Weigh the gold component rather than including stones as gold. Compare the same currency, karat and timestamp, and check whether you are seeing a buy or sell quote.
Check the quote before calculating
A displayed reference price is not a guaranteed executable offer. Read the source and update time, or enter an agreed custom price. For a broader checklist, see the gold karat value guide. Record the weight, unit, purity assumption and quoted price so you can explain why two estimates differ.