A karat measures the purity of gold based on a scale of 1-24, with 24 karats being the purest.

What this means is that 24-karat (24K) gold has 99.9% purity, generally referred to as “pure” gold.

18-karat (18K) gold has 75% pure gold. Its content is 18 parts gold and six parts other metal alloys.

14-karat (14K) gold has 58.3% pure gold. Its content is 14 parts gold and ten parts other metal alloys.

10-karat (10K) gold has only 41.7% pure gold in it and is the lowest purity that is still considered gold. Its content is 10 parts gold and 14 parts other metal alloys.

You can figure out these percentages of purity by dividing the number of karats by 24; for example, 14 divided by 24 equals 0.583 or 58.3%.

Why are there sometimes three numbers instead of the number of karats?

Internationally, you will also see three digits instead of karats stamped on jewelry which let you know the level of gold purity. This is called the millesimal fineness system. It indicates the purity of platinum, gold, and silver by parts per thousand of pure metal by mass in the alloy.

Gold

999.999 (The purest gold ever produced. Refined by the Perth Mint in 1958.)^[1]^[2]
999.99 (The purest type of gold in the market)
999.9
999 (Fineness equivalent to 24 karat, also known as three nines fine)
995 the minimum allowed in Good Delivery gold bars
990 = 99% pure also known as two nines fine
958.3 (equivalent to 23 karat)
916 (equivalent to 22 karat)
833 (equivalent to 20 karat)
750 = 75% pure (equivalent to 18 karat)
625 (equivalent to 15 karat)
585 (equivalent to 14 karat)
500 = 50% pure (equivalennt to 12 karat)
417 (equivalent to 10 karat)
375 (equivalent to 9 karat)
333 (equivalent to 8 karat; minimum standard for gold in Germany after 1884)

Silver

999.9 (Ultra-fine silver used by Royal Canadian Mint in the Canadian Silver Maple Leaf)
999 (Fine silver used in Good Delivery bullion bars, also known as three nines fine)
980 (common standard used in Mexico ca.1930 - 1945)
958 (equivalent to Britannia silver)
950 (equivalent to French 1st Standard)
925 (equivalent to Sterling silver)
900 (equivalent to Coin silver in the USA, also known as one nine fine)
833 (common standard used in continental silver especially among the Dutch, Swedish, and Germans)
830 (common standard used in older Scandinavian silver)
835 (a standard predominantly used in Germany after 1884)
800 (minimum standard for silver in Germany after 1884; Egyptian silver; Canadian silver circulating coinage)
750 (uncommon silver standard found in older German, Swiss and Austro-Hungarian silver)

Platinum

999 (also known as three nines fine)
995 (what most dealer would buy your platinum at if it is 100% pure; the most common purity for platinum bullion coins and bars)
950 (the most common purity for platinum jewellery)
900 (also known as one nine fine)
850

What is the Difference between a Karat and a Carat?

Karat

A karat measures the purity of gold based on a scale of 1-24, with 24 karats being the purest. And actually, below 10 karats isn’t even considered gold. In jewelry, you will often find the karat weight stamped on a clasp or inside a ring. It is frequently abbreviated as 18K.

Gold is a very soft metal, so it is fortified with alloys (a harder metal, such as copper, zinc, or nickel) to make it stronger. 24-karat gold has the least percentage of alloys, whereas 12-karat gold is only 50% pure, for example.

The United States and Canada use the word karat with a “K” whereas in other countries they might use a “C” or simply use the millesimal fineness system instead.

Carat

A carat measures the mass of gemstones and pearls when weighing them, which is why you have a one-carat diamond (1ct) in your 18-karat (18k) gold ring.

The word carat is derived from carob beans which had such a consistent weight that gemstones were measured against it for a very long time. However, in 1907, the carat became the legal standard for weighing precious gems.

The global standard is: one carat = 200 milligrams. It is based on the metric system. For example, if your diamond weighs one gram, it would be five carats. Below one carat in weight, it will be referred to as a fraction, such as a “quarter of a carat.”