Karat vs carat
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A carat is a weight. A karat is not. One carat is exactly 0.2 grams, and it measures gemstones. A karat, spelled with a k, measures how pure gold is, in twenty-fourths: 24 karat is pure gold, 18 karat is 75% gold.
They are a letter apart and mean entirely different things, which is survivable until you meet both in the same sentence. A one carat diamond in an 18 karat setting describes the weight of the stone and the purity of the metal, and neither number tells you anything about the other.
One is a mass, the other is a fraction
A weight, for stones
0.2 g exactly
Exact — CGPM, 1907
141.7476 carats make one standard ounce. It converts like any other unit of mass, because that is what it is.
A proportion, for gold
n ⁄ 24
A ratio, not a unit
There is no gram value for a karat, because it is not a quantity of anything. It says what fraction of a piece is gold.
Carat to grams and ounces
A carat converts, so here it is against everything else. Points are hundredths of a carat, and are how most stones are actually described.
| Carats | Points | Grams | Standard oz | Troy oz |
|---|---|---|---|---|
| 0.25 ct | 25 | 0.05 g | 0.00176 | 0.00161 |
| 0.5 ct | 50 | 0.1 g | 0.00353 | 0.00322 |
| 0.75 ct | 75 | 0.15 g | 0.00529 | 0.00482 |
| 1 ct | 100 | 0.2 g | 0.00705 | 0.00643 |
| 1.5 ct | 150 | 0.3 g | 0.01058 | 0.00965 |
| 2 ct | 200 | 0.4 g | 0.01411 | 0.01286 |
| 3 ct | 300 | 0.6 g | 0.02116 | 0.01929 |
| 5 ct | 500 | 1 g | 0.03527 | 0.03215 |
| 10 ct | 1,000 | 2 g | 0.07055 | 0.0643 |
A one carat diamond weighs 0.2 g. That is less than a fifth of the weight of a single gram of flour, and about 0.00705 of a standard ounce. Almost all of a ring's weight is the metal, not the stone.
Karat: what it changes is the gold inside
Karat does not change what a piece weighs. It changes how much of that weight is gold, which is the number that matters when you are buying or selling.
| Karat | Stamp | Purity | Gold in 10 g | Gold per standard oz | Gold per troy oz |
|---|---|---|---|---|---|
| 24K | 999 | 99.9% | 9.99 g | 28.321 g | 31.072 g |
| 22K | 916 | 91.67% | 9.167 g | 25.987 g | 28.512 g |
| 21K | 875 | 87.5% | 8.75 g | 24.806 g | 27.216 g |
| 18K | 750 | 75% | 7.5 g | 21.262 g | 23.328 g |
| 14K | 585 | 58.33% | 5.833 g | 16.537 g | 18.144 g |
| 10K | 417 | 41.67% | 4.167 g | 11.812 g | 12.96 g |
| 9K | 375 | 37.5% | 3.75 g | 10.631 g | 11.664 g |
This is why purity has to be part of any valuation. Ten grams of 14K gold holds 5.83 g of gold, not ten. Treating the whole weight as gold overstates it by 71%. Work out metal content →
Both at once, on one ring
Take a ring described as a one carat diamond in an 18 karat gold band, where the band weighs 5 grams.
- The stone weighs one carat, which is 0.2 g, or 0.00705 standard ounces.
- The band weighs 5 g, of which 18⁄24 is gold. That is 3.75 g of gold and 1.25 g of alloy.
- The whole ring weighs 5.2 g, or 0.1834 standard ounces.
Two numbers, two systems, no relationship between them. The carat figure tells you nothing about the gold, and the karat figure tells you nothing about the stone.
Which spelling to use
Both words descend from the carob seed, once used as a small standard weight. American English split them: karat for gold purity, carat for gemstone weight.
British English writes carat for both and relies on context, so a UK jeweller may write "18 carat gold" where an American writes "18 karat". Neither is wrong. If a listing says carat and is describing metal, it means purity; if it is describing a stone, it means weight.
Where a listing is genuinely ambiguous, the giveaway is the number. Purity runs 9 to 24. Stone weights are almost always under 5. A "22 carat" ring is about gold; a "0.75 carat" stone is about weight.
Frequently asked
What is the difference between karat and carat?
They measure different things entirely. A karat, spelled with a k, is a measure of gold purity: 24 karat is pure gold, and 18 karat is 18 parts gold out of 24, or 75%. A carat, spelled with a c, is a unit of weight for gemstones, equal to exactly 200 milligrams. One describes how pure a metal is, the other describes how heavy a stone is.
How many grams is one carat?
Exactly 0.2 grams, or 200 milligrams. This has been fixed by international agreement since 1907, which is why it is called the metric carat. In ounces that is 0.00705 standard ounces, or 0.00643 troy ounces.
Does karat measure weight?
No. Karat is a ratio, not a weight. A 10 gram ring in 14 karat and a 10 gram ring in 24 karat weigh exactly the same. The 24 karat ring simply contains more gold within those 10 grams: 9.99 grams against 5.83 grams.
How many carats are in an ounce?
A standard ounce holds 141.7476 carats, since a carat is 0.2 grams and a standard ounce is 28.349523125 grams. A troy ounce, used for the gold the stone is set in, holds 155.5174 carats.
Why are karat and carat spelled differently?
Both come from the carob seed, once used as a small standard weight. American English separated them, using karat for gold purity and carat for gemstone weight. British English writes carat for both and relies on context, which is why a UK jeweller may write "18 carat gold" where an American writes "18 karat".
What is a point in diamond weight?
One hundredth of a carat, so 2 milligrams. A stone described as 25 points weighs a quarter of a carat, or 0.05 grams. Jewellers use points because most stones sold weigh well under one carat.