Gold Purity Explained: Karats and Fineness
What does “karat” mean?
A karat is one part in 24. Pure gold is 24 karat, meaning 24 out of 24 parts are gold. 22 karat means 22 parts gold and 2 parts other metals. To turn karats into a percentage, divide by 24. So 22 ÷ 24 = 0.9167, or 91.67% gold.
The word is spelled “karat” in the United States. It is not the same as a “carat,” which measures the weight of gemstones. The Federal Trade Commission’s jewelry guides treat 24 karat as fine gold. They also say an item sold as “gold” should state its karat.
What is fineness?
Fineness is the same idea written as parts per thousand, or as a decimal. A coin marked .900 is 900 parts gold out of 1,000. A bar marked .9999 is 9,999 parts gold out of 10,000. Coin specs and bullion bars almost always use fineness rather than karats.
Traders sometimes say “three nines” for .999 and “four nines” for .9999. Both are sold as 24-karat gold. The difference is only in how small the trace of other metals is.
| Karat | Fineness | Gold content | Where you see it |
|---|---|---|---|
| 24k | .999 to .9999 | 99.9% to 99.99% | Bullion bars, American Gold Buffalo |
| 22k | .9167 | 91.67% | American Gold Eagle |
| 21.6k | .900 | 90% | U.S. gold coins, 1837–1933 |
| 18k | .750 | 75% | Fine jewelry |
| 14k | .583 | 58.3% | Jewelry |
| 10k | .417 | 41.7% | Jewelry; the usual floor for items marked “gold” under FTC guides |
Why isn’t every gold coin pure gold?
Pure gold is soft and scratches easily. Mixing in a little copper or silver makes it harder, so a coin holds its detail better when handled. That is why most historical gold coins were alloyed.
The U.S. Mint’s modern lineup shows both choices. The American Gold Eagle, first issued in 1986, is 22-karat gold: 91.67% gold, 3% silver and the rest copper. The American Gold Buffalo, first issued in 2006, was the first U.S. Mint coin struck in .9999 fine, 24-karat gold.
From 1837 until U.S. gold coinage ended in 1933, federal law set gold coins at 900 parts gold per 1,000. The rest was copper, with silver limited to no more than half of the alloy. Collectors often call these “pre-1933 gold.”
How do you calculate the gold content of a coin?
Multiply the coin’s total weight by its fineness. The result is the “fine weight,” sometimes listed as AGW (actual gold weight). Then divide grams by 31.1035 to get troy ounces.
- Start with the gross weight. A Saint-Gaudens $20 gold piece weighs 33.436 grams.
- Multiply by the fineness. 33.436 × 0.900 = 30.092 grams of gold.
- Convert to troy ounces. 30.092 ÷ 31.1035 = 0.9675 troy ounces.
The same math explains the one-ounce Gold Eagle. It weighs 33.931 grams. Multiply by 0.9167 and you get about 31.10 grams, which is one troy ounce of gold. The extra weight is the alloy. Our weight converter handles the gram-to-ounce step for you.
Gold content of common U.S. coins
| Coin | Fineness | Gross weight | Actual gold weight |
|---|---|---|---|
| American Gold Eagle, 1 oz ($50) | .9167 | 33.931 g | 1.0000 troy oz |
| American Gold Eagle, 1/2 oz ($25) | .9167 | 16.966 g | 0.5000 troy oz |
| American Gold Eagle, 1/4 oz ($10) | .9167 | 8.483 g | 0.2500 troy oz |
| American Gold Eagle, 1/10 oz ($5) | .9167 | 3.393 g | 0.1000 troy oz |
| American Gold Buffalo ($50) | .9999 | 31.103 g | 1 troy oz |
| Saint-Gaudens $20 (1907–1933) | .900 | 33.436 g | 0.9675 troy oz |
| Indian Head $10 (1907–1933) | .900 | 16.718 g | 0.4838 troy oz |
| Indian Head $5 (1908–1929) | .900 | 8.359 g | 0.2419 troy oz |
| Indian Head $2.50 (1908–1929) | .900 | 4.18 g | 0.1209 troy oz |
Gold Eagle weights come from the federal coinage law, and Gold Buffalo figures from the U.S. Mint. Pre-1933 figures come from NGC’s published specifications. Real coins can vary slightly within the Mint’s legal tolerance, and worn coins weigh a little less.
Does purity affect what a coin is worth?
Purity tells you how much gold a coin contains. It does not tell you the coin’s full value. A 22-karat Gold Eagle and a 24-karat Gold Buffalo of the same size hold the same amount of gold. For certified coins, grade, rarity, finish and label often matter as much as metal content. See our guides to NGC vs PCGS and coin labels.
Affinity Collectibles, an NGC Authorized Dealer and ANA member, can walk you through the specs of any certified gold coin. Questions are welcome through our contact page.
Frequently asked questions
Is 22-karat gold “real” gold?
Yes. It is 91.67% gold with a small amount of silver and copper added for durability. A one-ounce Gold Eagle contains a full troy ounce of gold.
What is the difference between .999 and .9999 gold?
.999 is 99.9% gold and .9999 is 99.99% gold. Both are considered 24 karat. The difference is a tiny amount of trace metal.
Why is a pre-1933 $20 gold piece not a full ounce of gold?
It was made to a legal weight in grains, not to a round troy ounce. At .900 fine, its gold content works out to 0.9675 troy ounces.
Is karat the same as carat?
No. Karat measures gold purity. Carat measures the weight of gemstones.
Sources
- 31 U.S. Code § 5112: Denominations, specifications, and design of coins
- U.S. Mint: Bullion Coin Programs
- U.S. Mint: American Eagle Coin Program
- U.S. Mint: First pure gold U.S. coins (2006)
- U.S. Mint: Act of January 18, 1837
- NGC: U.S. Gold Coin Melt Values (specifications)
- FTC Guides for the Jewelry, Precious Metals, and Pewter Industries (16 CFR Part 23)