Carat Weight Calculator

For a round brilliant the formula is diameter squared times depth times 0.0061, with the diameter and depth both in millimetres. A 6.5 mm by 4.0 mm stone gives 6.5 x 6.5 x 4.0 x 0.0061, which is about 1.0309 carats. It is an estimate, not a substitute for a calibrated scale.

Use the average girdle diameter across the widest part of the stone and the total depth from the table on top down to the culet at the bottom. If the diameter varies, average the smallest and largest readings before entering it.

One metric carat equals exactly 0.2 grams, which is also 200 milligrams. So 5 carats weigh exactly 1 gram, and half a gram is 2.5 carats.

Carats and grams are rounded to four decimal places and milligrams to two. That is finer than most jewellery scales, so the displayed precision reflects the maths, not a claim about measuring accuracy.

The 0.0061 coefficient is tuned for round brilliants. Ovals, princess, cushion and other cuts use different coefficients and length-to-width adjustments, so the round formula only gives a rough idea for them.

A thick girdle, a deep pavilion or a bulging culet all add mass that a plain diameter and depth formula cannot see, so a heavy make can outweigh the estimate. A shallow, spread cut can weigh a little less.

Yes. One carat is divided into 100 points, so a 0.25 carat diamond is a 25 point stone and a 0.5 carat stone is 50 points. This tool shows the decimal carat figure rather than points.

The unit conversion between carats, grams and milligrams works for any gem, since the carat is a fixed unit of mass. The dimension estimate assumes diamond density and a round brilliant cut, so it is not reliable for stones like ruby or sapphire.

No. The calculation runs entirely in your browser, so your measurements and weights never leave your device.