Troy ounces, grams and purity: how gold is weighed and measured
Gold is weighed in troy ounces and grams, and its purity is given in parts per thousand or karats. This guide explains the units, shows a conversion table and explains why a 22-karat coin such as the American Eagle still contains a full ounce of fine gold.
The short answer
Precious metals are weighed in troy ounces, not the ordinary ounces used for groceries. One troy ounce is 31.1035 g, about 10% heavier than an ordinary ounce. Bars are also made in grams and kilograms.
Purity, or fineness, is given in parts per thousand: 999.9 means 99.99% gold. Coins made of 22-karat gold (916.7), such as the American Eagle, weigh more than an ounce in total, but still contain one full ounce of pure gold. That pure content, the fine weight, is what the price of a coin or bar is based on.
Troy ounces, ordinary ounces and grams
There are two kinds of ounce in common use:
- The troy ounce (oz t) is 31.1035 g. It is the standard unit for gold and silver, and wholesale metal prices are normally quoted in US dollars per troy ounce.
- The ordinary or avoirdupois ounce (oz) is 28.3495 g. It is the ounce used for everyday goods such as food and mail.
So when a coin is described as "1 oz", it means one troy ounce. One kilogram is about 32.15 troy ounces. Note that the troy pound (12 troy ounces) is lighter than the ordinary pound (16 ordinary ounces), which is one reason the trade sticks to ounces and grams.
| Weight | Troy ounces | Ordinary ounces |
|---|---|---|
| 1 g | 0.0322 oz t | 0.0353 oz |
| 5 g | 0.1608 oz t | 0.1764 oz |
| 10 g | 0.3215 oz t | 0.3527 oz |
| 20 g | 0.6430 oz t | 0.7055 oz |
| 31.1035 g (1 oz t) | 1.0000 oz t | 1.0971 oz |
| 50 g | 1.6075 oz t | 1.7637 oz |
| 100 g | 3.2151 oz t | 3.5274 oz |
| 1 kg | 32.1507 oz t | 35.2740 oz |
If you weigh a 1 oz coin on a kitchen scale set to ounces, it will show about 1.1 oz, not 1. That is not a sign of a problem: the scale is using the ordinary ounce.
Fineness and karats
Purity is shown in two ways. Fineness gives the parts of pure metal per thousand. Karats (k or kt) divide gold into 24 parts, so 24 karat is pure gold and 22 karat is 22 parts gold out of 24.
| Fineness | Karats | Gold content | Typical examples |
|---|---|---|---|
| 999.9 | 24 k | 99.99% | Maple Leaf, Britannia (since 2013), Vienna Philharmonic, American Buffalo, most bars |
| 999 | 24 k | 99.9% | Some bars and coins |
| 916.7 | 22 k | 91.67% | American Eagle, Krugerrand |
| 900 | about 21.6 k | 90% | US gold coins struck before 1933, such as the $20 Double Eagle |
The United States Mint makes two well-known 1 oz gold bullion coins. The American Eagle is 22-karat gold and contains one troy ounce of fine gold. The American Buffalo is 24-karat (999.9) gold, also containing one troy ounce. AU Trading sells the American Eagle; the Buffalo is mentioned here only as an example.
"Karat" for gold purity is different from the "carat" used to weigh diamonds, which is a unit of weight.
Why a 22-karat coin still holds a full ounce
A 22-karat coin is not "less gold" than a 24-karat coin of the same denomination. The mint simply adds other metals on top of the full ounce of gold.
The American Eagle is a good example. The 1 oz coin weighs about 33.93 g in total, its gross weight. At 916.7 fineness, 31.10 g of that is pure gold, exactly one troy ounce. The rest is a mix of silver and copper, which makes the coin harder and more resistant to scratches and gives it its warm color. The South African Krugerrand uses the same idea with an alloy of mainly copper, at the same gross weight.
What this means in practice:
- A 1 oz American Eagle and a 1 oz Maple Leaf contain the same amount of gold.
- The extra alloy has no meaningful value in the price. You pay for the gold.
- 22-karat coins are tougher in handling; 24-karat coins scratch and dent more easily.
What "fine weight" means on AU Trading
Each product page on AU Trading shows the weight, the purity and the premium over the market price. The fine weight is the amount of pure metal the piece contains, as distinct from its gross weight.
Prices are based on fine weight because that is what the wholesale market trades. The live price of each product, in US dollars, is the spot price of its fine metal content plus the premium shown on its page, refreshed every minute. So the price of an American Eagle is worked out on 31.10 g of gold, not on its 33.93 g gross weight.
When comparing products, compare fine weights and premiums, not gross weights. You can follow the metal price itself on the gold price chart. This guide isn't investment advice.
How to tell at a glance
You can usually see what you're holding without any equipment:
- Bars are stamped with the refiner's name or mark, the weight, the fineness and, for most bars, a serial number.
- Assay certificates come with many bars, often as a sealed card. The certificate states the weight and fineness, and its serial number should match the bar. Keep the card intact; many dealers prefer bars still in their original packaging.
- Modern bullion coins usually state their fine weight on the coin itself. The American Eagle, for example, reads "1 OZ. FINE GOLD". Older coins, such as pre-1933 US gold coins, don't, so it helps to know their specification.
- Mint marks and design details identify where and when a coin was struck. The issuing mint publishes the official weight, diameter and thickness of each coin.
Quality marks on jewelry follow their own rules and don't usually apply to bullion coins.
Spotting fakes
Counterfeit coins and bars do exist, especially among popular products. Some simple checks help:
- Weight. Use an accurate scale that reads to at least 0.01 g, set to grams. A coin that is noticeably off its official gross weight is a warning sign.
- Dimensions. Measure the diameter and thickness with calipers and compare them with the mint's figures. Because gold is very dense, a fake made to the right weight often ends up the wrong size.
- Look and sound. Compare the details and finish with a known genuine piece or the mint's own images.
None of these checks is conclusive alone, and a well-made fake can pass some of them. The most reliable protection is to buy from an established, reputable dealer, and be cautious of prices well below the market. If you are buying for the first time, our guide on how to buy gold online walks through the process.
Common questions
Is a troy ounce the same as an ounce?
No. A troy ounce is 31.1035 g and an ordinary ounce is 28.3495 g. In precious metals, "oz" always means the troy ounce.
Is 24-karat gold better than 22-karat gold?
Not for value. A 1 oz coin contains one ounce of fine gold whether it is 22 or 24 karat. The difference is mainly in hardness and appearance, and which coins you prefer.
Why is my coin heavier than its stated weight?
The stated weight on a 22-karat coin is its fine weight. The gross weight includes the alloy, so a 1 oz American Eagle or Krugerrand weighs about 33.93 g in total.
Is the American Eagle pure gold?
No, it is 22-karat gold (916.7 fine), but it contains a full troy ounce of pure gold. If you prefer 24-karat coins, the Maple Leaf is 999.9 fine.
