The short answer
For a homogeneous 18K gold alloy, the nominal gold fraction is 18 ÷ 24 = 75%. Apply it to the relevant metal weight, not blindly to an entire ornament.
What the karat mark tells you
The FTC explains that karat marks describe the proportion of gold in a gold alloy: an 18K item contains 18 parts gold and six parts other metal; 14K contains 14 parts gold and ten parts other metal. The total is 24 parts. These are composition descriptions, not a quote from a buyer.[2]
Use nominal purity as a calculation input only when it describes the relevant material. A stamp is not an independent assay. If authenticity, repairs or composition are uncertain, the uncertainty belongs in the result rather than disappearing behind two decimal places.
Choose the right weight
Write the formula as eligible alloy mass × gold fraction × reference price per unit of fine gold. The three terms must use consistent units. If your scale measures grams, a price per gram avoids accidentally mixing ordinary ounces with troy ounces.
Do not include gemstones or other non-alloy components in the mass merely because they are on the scale. Hollow construction also needs care: an object can be physically large without being heavy. The calculation starts with relevant metal mass, not appearance or original purchase price.
A transparent example
Hypothetical inputs: 10 grams of relevant 18K alloy and a reference price of 100 currency units per gram of fine gold. Fine-gold content is 10 × 0.75 = 7.5 grams. The theoretical gold-content value is 7.5 × 100 = 750 currency units. The reference price of 100 is invented solely for arithmetic, not presented as market data.
If the 10-gram gross weight includes a 2-gram stone, the eligible alloy is only 8 grams under that assumption. Fine-gold content becomes 6 grams and the same hypothetical reference yields 600. The difference comes from the mass assumption, not a change in the gold market.
Why plating breaks the shortcut
The FTC distinguishes gold-filled, overlay and plated items from gold alloys and describes vermeil as sterling silver coated or plated with gold. A surface label must not be applied as if the entire object had that gold fraction.[2]
For a plated object, multiplying total weight by 18/24 can dramatically overstate gold content. Without reliable coating mass or appropriate testing, stop the calculation. “Insufficient composition information” is a more useful result than a confidently wrong valuation.
What the result can and cannot do
The calculation isolates one component: theoretical gold content at an explicitly chosen reference. It excludes workmanship, brand, stones, collectible value, taxes, fees and the terms of a buyer’s offer. It is neither a guaranteed floor nor an appraisal.
To compare an actual offer, ask which weight and purity the buyer used, what deductions apply, and how long the offer remains valid. Keep the written terms. FTC labeling guidance here concerns the United States; do not treat it as the complete law in another jurisdiction.[2]
Sources & scope
Links support definitions and methodology. Worked examples are hypothetical, not quotes; the review date is not the observation date of a market value.