Convert diamond carat weight to grams and points, and estimate round-brilliant diameter from carat (or carat from diameter).
Result
How to use this tool
Pick carat to size or diameter to carat.
Enter the known value.
Read grams, points, diameter and carat together.
Treat diameter as an approximation for well-cut rounds.
Formula & worked example
A 2 ct round brilliant weighs 0.4 g (200 points) and measures about 8.19 mm across.
Limitations — read this before you rely on it
Diameter estimate is a round-brilliant rule of thumb only; it is not a grading measurement and ignores cut, girdle and shape.
Frequently asked questions
Exactly 0.2 grams, so 5 carats make 1 gram.
One hundredth of a carat: a 50-point stone is 0.50 ct.
Weight grows with volume, which scales as diameter cubed for a fixed shape and cut, so diameter scales as the cube root of weight.
Only approximately for round brilliants; princess, oval and other shapes have different length-to-width ratios at the same weight.
No — cut, colour and clarity matter as much, and price per carat rises steeply with size.