Diamond carat to mm converter
Convert a round diamond’s carat weight to its diameter in millimeters, or measure the stone and estimate its carats.
That's about the same as a one-carat diamond. A well-cut round brilliant 6.454 mm across weighs about 1 ct.
What it means
How big is that stone?
For well-cut round brilliants. Deep stones weigh more for their spread and shallow ones less; other shapes and gems differ.
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That diamond in every unit
Learn
Three things worth knowing
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6.5 mm is one carat
Half a carat is about 5.2 mm, a carat 6.5 mm, two carats 8.2 mm. Doubling the weight adds only a quarter to the width.
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Carob seeds
Traders once balanced gems against carob seeds, thought to weigh the same each. The metric carat of exactly 200 mg was agreed in 1907; a point is a hundredth of a carat.
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Carat is not karat
Carat (ct) weighs gems. Karat (K) is the gold purity of the ring around it: 18K is 75 % gold.
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Background
About carats and size
A carat is a weight, 0.2 grams, not a size. What you see is the stone’s spread, so jewellers estimate one from the other. For a well-cut round brilliant, about 61 % as deep as it is wide, the weight grows with the cube of the diameter: carats ≈ 0.0037 × d³.
That is why a 2 carat stone is only 26 % wider than a 1 carat, not twice as wide.