How this calculator works
A diamond weighs a little over three and a half times the same volume of water, and a metric carat is exactly 0.2 grams. For a round brilliant, the trade uses a compact formula that combines the average girdle diameter squared, the total depth and a constant of 0.0061. The constant folds in the density, the rounding of the cut and the usual proportions of a well-made stone. Fancy shapes use their own constants with length times width times depth, because an oval, a princess or a marquise does not fill its bounding box the same way a round does. The result is an estimate from measurements, not a certificate weight; only a gemological laboratory or a jeweller's scale can give the exact carat weight.
Formula
round: carats = diameter^2 x total depth x 0.0061. fancy shapes: carats = length x width x total depth x the shape constant. The constants are 0.0062 for oval, 0.00815 for cushion, 0.0080 for emerald and asscher, 0.0059 for heart, 0.0083 for princess, 0.00615 for pear, 0.00565 for marquise, 0.0081 for radiant, 0.0057 for trillion and 0.00915 for baguette. Multiply the result by one plus the correction share. grams = carats x 0.2.
Worked example
A round brilliant 6.5 mm across and 4.0 mm deep is 6.5 x 6.5 x 4.0 x 0.0061 = 1.031 carats, or about 0.206 grams. An oval 8 mm by 6 mm and 4 mm deep is 8 x 6 x 4 x 0.0062 = 1.190 carats. An emerald cut with the same 8 by 6 by 4 measurements is 8 x 6 x 4 x 0.0080 = 1.536 carats, because a step cut fills more of its outline.
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Frequently asked questions
How accurate is a measured carat estimate?
For a well-proportioned stone the usual spread is about ten percent, and it can be worse for a thick girdle, a high crown or a deep pavilion. A one carat stone might weigh 0.9 or 1.1 carats. Use the estimate for budgeting and comparison, not as a substitute for a certificate.
Why does a round diamond use diameter but other shapes use length and width?
A round brilliant is close to circular, so one diameter describes its spread. Fancy shapes are elongated or squared off, and their length-to-width ratio matters, so both measurements are needed. The shape constant then accounts for how much of the rectangular outline the cut actually fills.
What is the difference between carat and karat?
Carat with a c is a unit of mass, equal to 0.2 grams. Karat with a k is a measure of gold purity, where 24 karat is pure gold. The two words sound alike but measure completely different things.
Should I use the average diameter?
For a round stone, yes. Measure the smallest and largest girdle diameters and average them, because a slightly out-of-round stone can differ by a few tenths of a millimetre. Using only the largest diameter overstates the weight.