Gold measurement

Gold measurement could refer to two different aspects of an item; purity and weight. Both are vital measurements in working out what a gold item is worth, with the physical size being less accurate as an indicator.

The weight of gold is most commonly measured in a unit known as troy ounces, although in a world where the metric system is growing in adoption, grams are also commonly used. Gold purity is measured in units of carat or fineness.

Gold purity

100% pure gold is extremely soft and malleable, so for most practical purposes it is usually mixed (alloyed) with a harder metal. Items described as gold are, in most cases, a gold alloy and not pure gold. The purity of metal is usually measured in carats or fineness, and these are also often quoted as a percentage.

Carat measures the amount of pure gold to other metals in 24 parts. 24 carat is therefore pure gold, and 9 carat would equate to 9 parts gold, to 15 parts of another metal.

Fineness measures the amount of pure gold to other metals in parts per thousand. 999.999 fineness is pure gold. 9 carat for example is equal to 375 fineness; that is 375 parts gold to 625 parts of another metal.

Carat in the US is a unit of measurement used only for gemstones, not gold. In the US carat is spelt karat when measuring the purity of gold. In Britain, 9 carat is the lowest acceptable purity used in manufacture, Germany and other European nations accept 8 carat, whilst in the US the minimum is 10 carat.

Carat Fineness Percentage
24 999.99 100%
22 916 91.6%
18 750 75%
15 625 62.5%
14 585 58.5%
10 417 41.7%
9 375 37.5%
8 333 33.3%

Gold weight

Gold is often weighed in grams, but the troy ounce is the measurement most commonly used for gold bullion coins.

A troy ounce should not be confused with the common or ‘avoirdupois’ ounce. A troy ounce is equal to 31.1035 grams, whereas a standard ounce is lighter at 28.3495 grams. That is 2.7539 grams less.

The Weights and Measures Act of 1878 abolished both the historic British pennyweight and the troy pound. The troy ounce, however, remains an official British unit of weight.

The Tola is another pre-decimal unit of weight that has historic roots in Asia. It is equal to 11.6638038 grams and is commonly used today as a measure of gold, particularly in India, and several gold manufacturers produce gold bars in Tola measurements.

Graphic comparing the three main weight measurements for gold.

Gold bar weight

Banks, central banks, and bullion investors generally deal in 24 carat, 999 fineness gold bars. In the ‘Good Delivery’ specifications the London Bullion Market Association defines that a standard bar should be 400 troy ounces. These 400 ounce bars measure between 210 to 290 mm in length, 55–85 mm in width, and 25–45 mm high.

With a theoretical gold price of £1,000 per ounce (which in reality is higher at the time of writing), this would make each LBMA standard bar worth £400,000. Private bullion investors will therefore generally deal in more affordable smaller sizes. Common bullion bar sizes are 1 kilo, 10 oz, 100 g, 50g, 1 oz, 20g, 10g, 5g, 2.5g and 1g.

Graphic showing the different sizes of gold bars.

Gold Coins

Whilst bullion bars are generally all pure, 24 carat gold, bullion coins can also be minted from alloys to improve their durability.

The quoted weight of a coin is still usually its pure gold content. For example, a one ounce gold Krugerrand weighs in total 33.93 gram; 2.827 grams more than a troy ounce. That is one troy ounce, or 31.103 grams, of pure gold, plus 2.827 grams of another metal (copper in the case of the Krugerrand). It is the troy ounce of gold however that is of value and the measurement is therefore given as an ounce.

When selling gold to a bullion dealer, such as BullionByPost, it is therefore best to have the two key gold measurements - weight and purity. This will ensure you get a quick and accurate quote. When weighing gold it is easiest to use grams as this is standard across all markets. Purity is difficult to determine without specialist equipment, but you may find gold hallmarks which will give you a good indication.

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