Gemstones
Diamond MM to Carat Calculator (All Shapes)
Estimate a diamond’s carat weight from its length, width and depth in mm for round, oval, princess, pear and more. An estimate, not a weighing.
Result
Your result
- Estimated weight
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- Formula used
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Choose a shape and enter the measurements in mm.
How to use it
- Choose the shape of the stone.
- Measure length, width and depth with a gauge, in mm.
- Read the estimated carat weight.
Uses widely used estimation factors: round = diameter² × depth × 0.0061; oval 0.0062; princess 0.0083; cushion and radiant 0.0081; emerald 0.0080; pear and heart 0.0059; marquise 0.00565 (length × width × depth × factor).
Girdle thickness, bulges and cut proportions change the real weight. Only a scale gives the true carat weight, and carat says nothing about quality.
Frequently asked questions
Questions about this tool
How do you estimate carat weight from mm?
Multiply the measurements by a shape factor. For a round diamond: diameter × diameter × depth × 0.0061. A 6.5 × 6.5 × 3.95 mm round is about 1.02 ct.
Does this work for lab-grown diamonds and moissanite?
Lab-grown diamonds have the same density as natural ones, so the factors apply. Moissanite is lighter than diamond, so the same size weighs less.