How this calculator works
For a well-proportioned round brilliant, the trade estimates weight from the average girdle diameter, the total depth and a constant of 0.0061. The diameter is squared, the depth is measured as a share of the diameter, and the result is in carats when both lengths are in millimetres. A typical round brilliant has a total depth near 61 or 62 percent of its diameter; a deeper stone of the same diameter weighs more, and a shallower one weighs less. The estimate describes the cut and the density of diamond, not a particular stone. A thick girdle, a high crown or an unusual pavilion can move the true weight by ten percent or more, so use the figure for comparison and budgeting rather than as a certificate weight.
Formula
carats = diameter^2 x (depth share x diameter) x 0.0061, with the diameter in millimetres. Equivalently carats = 0.0061 x depth share x diameter^3. In reverse, diameter = (carats / (0.0061 x depth share))^(1/3). Total depth in millimetres = diameter x depth share.
Worked example
A round brilliant 6.5 millimetres across with a 61.5 percent total depth is 6.5 x 6.5 x 4.0 x 0.0061, about 1.030 carats, and its depth is 6.5 x 0.615 = 4.0 millimetres. A 4.0 millimetre stone at the same depth is 0.0061 x 0.615 x 64 = 0.240 carats. Going the other way, a 1 carat stone at 61.5 percent depth has a diameter of about 6.44 millimetres, which is why a one carat round is often quoted as roughly 6.5 millimetres across.
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Frequently asked questions
Is a 1 carat diamond always 6.5 mm?
No. The usual range is about 6.3 to 6.5 millimetres for a well-cut round brilliant, but a deep stone can be 6.0 millimetres and a shallow one can be 6.7 millimetres. Diameter and weight are linked through the depth and the cut, not by a single fixed number.
Why does the depth percentage matter?
The formula multiplies the area of the girdle by the total depth. A stone with a larger depth share has more material below the girdle for the same diameter, so it weighs more. The percentage is the total depth divided by the average diameter, times 100.
Does this work for oval, princess or emerald cuts?
Not directly. Fancy shapes need their own constants and both a length and a width. The diamond weight calculator applies the standard constants for those shapes; this page is specific to round brilliants.
How accurate is the table?
It is a trade estimate for a typical well-cut stone. Expect about ten percent either way for an unusual cut, a thick girdle or a strongly domed crown. The table is most reliable for comparing stones of similar make, not for predicting an exact certificate weight.