Carat Converter
Carats to grams, points and grains, plus diamond size in mm.
About this tool
Enter a weight in carats or any other unit to see the rest. For round brilliant diamonds the tool also estimates the diameter in millimetres from the carat weight, and the carat weight from a measured diameter.
Carats, points and diamond size
The metric carat, adopted internationally in 1907, is exactly 200 milligrams, so 5 carats make 1 gram. It's divided into 100 points: a 0.25 ct stone is 25 points. Gem dealers also meet grains (1 ct is about 3.1 grains) and troy ounces (155.5 ct). For round brilliant diamonds with typical proportions, weight is roughly 0.0037 Γ diameterΒ³ (mm), so 0.5 ct is about 5.1 mm across, 1 ct about 6.5 mm and 2 ct about 8.1 mm. Doubling the weight increases the width by only about 26%. Honest limits: the size estimate applies to round diamonds only; other cuts and gems differ (sapphires and rubies are denser, so a 1 ct sapphire is smaller than a 1 ct diamond), and deep or shallow cuts change the spread. Carat is weight, not quality: price also depends on cut, colour and clarity. Don't confuse carat with karat, which measures gold purity. The ring size converter and the unit converter are related.
Frequently asked questions
How many grams is 1 carat?
Exactly 0.2 grams (200 milligrams).
How big is a 1 carat diamond?
A well-cut round brilliant is about 6.5 mm across.
What is a point in diamond weight?
One hundredth of a carat. A 50-point diamond is 0.5 ct.
Is carat the same as karat?
No. Carat measures gemstone weight; karat measures gold purity (24K is pure gold).
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.