Boneyard Tools

How to estimate diamond carat weight from measurements

Where the diameter squared times depth times 0.0061 formula comes from, how to measure a stone, and why a real weight can differ.

Open the Carat Weight Calculator →

The round brilliant weight formula

The estimate treats a round brilliant as a solid of revolution whose volume scales with the square of its diameter and its total depth. Multiplying diameter squared by depth gives a number proportional to that volume, and the constant 0.0061 folds in the density of diamond and the average proportions of a well-cut brilliant so the result comes out in carats. It is the same Tolkowsky-style rule cutters have used for decades to gauge a stone without unmounting it. The formula assumes all measurements are in millimetres.

Measuring diameter and depth

Diameter is the distance across the girdle, the widest ring around the stone. Because a real brilliant is never perfectly round, take the smallest and largest readings with a millimetre gauge or calipers and average them. Depth is the vertical distance from the flat table on top to the pointed culet at the bottom. Careful measurement matters because diameter is squared in the formula, so a tenth of a millimetre error on diameter moves the estimate more than the same error on depth.

Why the real weight can differ

The formula captures an average make, but individual stones vary. A thick girdle adds a band of extra material around the edge, a deep or bulged pavilion adds mass below the girdle, and a large culet adds a little at the very bottom. All three push the true weight above the estimate. A spread cut that trades depth for face-up size can weigh slightly less than the formula suggests. Treat the result as a guide for a mounted stone you cannot put on a scale, not as a certified weight.

Carats, grams and points

The metric carat is a fixed unit: exactly 0.2 grams, or 200 milligrams. That makes unit conversion pure arithmetic, so 2.5 carats is always 0.5 grams and 250 milligrams is always 1.25 carats regardless of the stone. Jewellers also split each carat into 100 points for small stones, so a 0.05 carat accent is a five pointer. This calculator reports the decimal carat weight alongside grams and milligrams so you can move between trade and metric units in one step.

Frequently asked questions

Do I measure in millimetres or inches?

Millimetres. The 0.0061 coefficient is calibrated for diameter and depth in millimetres, so convert any inch readings to millimetres before entering them.

Why is diameter squared but depth is not?

The face of a round stone is a circle whose area grows with the square of its diameter, while depth adds a single linear dimension. Multiplying the two gives a volume-like figure, so diameter has a much larger effect on the estimate.

Can I reverse it to find a diameter for a target weight?

Not directly. This tool solves weight from dimensions or converts between units, so you would rearrange the formula by hand to size a stone for a chosen carat weight.