Carats to Grams Converter

One metric carat is exactly 0.2 grams — convert gemstone weights to grams, milligrams and ounces.

Conversion table

Weight	In grams
0.5	0.1 g
1	0.2 g
1.5	0.3 g
2	0.4 g
2.5	0.5 g
3	0.6 g
4	0.8 g
5	1 g
6	1.2 g
7	1.4 g
8	1.6 g
9	1.8 g
10	2 g
12	2.4 g
15	3 g

💠 Weights to know

1 carat0.2 g exactly
1 gram5 carats
0.50 ct100 mg
1 point0.01 ct = 2 mg

Reading a gem certificate

Carat is weight, karat is gold purity. A 1-carat diamond in an 18-karat setting mixes two unrelated scales that happen to share a name in most languages.
Under one carat, the trade counts points: 0.75 ct is "seventy-five points", and a 0.01 ct chip is one point.
Certificates state weight to two decimals, and the rule is to round down. A stone measured at 0.999 ct is sold as 0.99, not 1.00.
Weight is not size. A 1 ct ruby looks smaller than a 1 ct diamond because ruby is denser, so the same mass occupies less space.

Why the carat is exactly 0.2 grams

The word comes from the carob seed, once used as a counterweight on jewellers' scales across the Mediterranean. The story that carob seeds are unusually uniform is a myth — they vary as much as any seed — and that is precisely why the unit had to be fixed. Until the twentieth century, the carat differed from city to city, so a stone changed weight when it crossed a border. The metric carat of 200 milligrams was agreed internationally in 1907 and adopted through the following years, which is why the conversion is exact rather than approximate: 1 ct = 0.2 g = 200 mg, and 1 g = 5 ct.

Carat weight versus karat purity

These are two different measurements that sound identical. Carat with a C is the weight of a gemstone: 0.2 grams. Karat with a K is the purity of a gold alloy, expressed in twenty-fourths — 24 karat is pure gold, 18 karat is 18 parts gold in 24, or 75%, and 14 karat is 58.3%. English separates the two spellings; Spanish, French, Portuguese and German use one word for both, so a ring described as "18 carats" is almost always about the gold, not the stone. If a number above 24 appears, it is certainly weight; if it is 9, 14, 18 or 22, it is almost certainly purity.

How the trade uses the number

Price per carat rises with size, so a 2 ct stone costs far more than two 1 ct stones of the same quality — the weight doubles and the rate per carat rises as well.
"Magic sizes" at 0.50, 0.75, 1.00 and 2.00 ct carry a price jump. A 0.98 ct stone can cost noticeably less than a 1.01 ct one that looks the same.
Total carat weight (tcw) on a multi-stone ring adds every stone together, so 1 ctw can be thirty tiny stones rather than one.
Loose stones are weighed; mounted stones are usually estimated from measurements, because removing them to weigh risks damage.
Pearls are often sold by momme, a Japanese unit of 3.75 g, which is 18.75 carats.
Diamond scales used in the trade read to 0.001 ct, that is one microgram short of two milligrams of resolution.

Weight, size and what the eye sees

A well-cut 1 ct round brilliant diamond measures about 6.4–6.5 mm across the top.
A 0.50 ct round is roughly 5.2 mm and a 2 ct about 8.1 mm — doubling the weight adds only a quarter to the diameter.
Denser gems look smaller for the same weight: ruby and sapphire are about 4.0 g/cm³ against diamond's 3.52.
Emerald is less dense than diamond, so a 1 ct emerald looks slightly larger than a 1 ct diamond.
A deep-cut stone hides weight below the girdle: it weighs more than it looks, which is why cut grade matters to value.
For everyday scale: a carat is about the weight of a small paperclip's tip, and five of them make one gram.

Worked examples

Both directions, and the point scale

A 1.25 ct diamond

1.25 ct × 0.2 g = 0.25 g
0.25 g = 250 mg
In trade language: one carat and twenty-five points

1.25 ct = 0.25 g = 250 mg

A stone that weighs 0.6 g

0.6 g ÷ 0.2 g per carat = 3 ct
3 ct is 300 points
Check the other way: 3 × 0.2 = 0.6 g

0.6 g = 3.00 ct

Frequently Asked Questions

How many grams is 1 carat?

Exactly 0.2 grams, or 200 milligrams. The metric carat was fixed at that value internationally in 1907, so the conversion carries no rounding at all: five carats make exactly one gram, and a 1.5 ct stone weighs 0.3 g.

What is the difference between carat and karat?

Carat is the weight of a gemstone — 0.2 grams. Karat is the purity of gold, measured in twenty-fourths: 24 karat is pure, 18 karat is 75% gold, 14 karat is 58.3%. English uses different spellings; most other languages use one word, so context decides. A ring that is "18 carats" is describing its metal.

What is a point in diamond weight?

One hundredth of a carat, so 2 milligrams. Stones under one carat are usually described in points: a 0.30 ct diamond is "thirty points", and 0.75 ct is "seventy-five points". Above a carat, the trade goes back to decimals.

Does a bigger carat weight always mean a bigger-looking stone?

No. Weight and appearance separate in two ways. Denser gems pack the same mass into less volume, so a 1 ct ruby looks smaller than a 1 ct diamond. And a stone cut too deep carries weight under the girdle where nobody sees it, which is why two 1 ct diamonds can differ visibly in face-up size.

Why do diamonds jump in price at exactly 1.00 carat?

Because demand clusters at round numbers. Buyers ask for a one-carat stone, so the market prices 1.00 ct well above 0.95 ct even though the difference is 10 milligrams and invisible. Buying just under a magic size — 0.90 or 1.90 — is the standard way to get the same look for less.

How is total carat weight different from carat weight?

Carat weight describes one stone; total carat weight, written ctw or tcw, adds up every stone in a piece. A ring advertised at 1 ctw might hold a 0.5 ct centre and forty small accent stones, and it is worth much less than a single 1 ct diamond of the same quality.