Diamond weight calculator: carat weight from mm
By HeyGema™ · Updated
In short
Multiply length, width and depth in mm by a constant for the shape to estimate a diamond's weight in carats. A round 6.5 x 6.5 x 3.9 mm stone comes to about 1 carat. The estimate is good to a few percent; only a scale gives the exact weight.
Diamond weight calculator
About 1.01 carat
Round: diameter x diameter x depth x 0.0061. An estimate: a thick or thin girdle, or a deep or shallow cut, moves it by a few percent. A grading report weighs the stone.
The formula for each shape
Diamond weight in carats is about length x width x depth x a constant that depends on the cut. A round stone uses its diameter twice. For another gem, multiply by its specific gravity and divide by 3.52.
- Round
- Formula: diameter x diameter x depth x 0.0061
- Oval
- Formula: length x width x depth x 0.0062
- Cushion
- Formula: length x width x depth x 0.008
- Princess
- Formula: length x width x depth x 0.0083
- Emerald cut
- Formula: length x width x depth x 0.0094
- Asscher
- Formula: length x width x depth x 0.0087
- Radiant
- Formula: length x width x depth x 0.008
- Pear
- Formula: length x width x depth x 0.00615
- Marquise
- Formula: length x width x depth x 0.00565
- Heart
- Formula: length x width x depth x 0.0059
- Trillion
- Formula: length x width x depth x 0.0057
- Baguette
- Formula: length x width x depth x 0.009
How to measure the three numbers
Length and width are the longest and widest points across the top; depth runs from the flat top (table) to the bottom point (culet). A sliding caliper reads to 0.01 mm. Round stones are measured at the narrowest and widest diameter and averaged. See how to measure the size of a gem. If you cannot measure depth, a round brilliant is usually about 60% as deep as it is wide.
How accurate it is
The constants come from the estimation tables used in the trade and were checked against stones listed with their measurements and weights. Expect a few percent either way: a thick girdle or an unusual shape adds weight the formula does not see. See the methodology.