Gemstone Weight Calculator
Estimate carat weight from measurements.
Estimate a gemstone's carat weight from its length, width and depth, using its specific gravity and a shape factor.
Estimated weight
1.54 ct
±10% · 0.307 g
This is an estimate, not a weight. It assumes well-proportioned cutting, and depth is both the hardest measurement to take on a set stone and the one the result is most sensitive to. Treat it as a range rather than a figure.
How the Gemstone Weight Calculator works
A stone already set in a piece cannot be put on a scale, so its weight has to be estimated from what can be measured. Length times width times depth gives a bounding box; a shape factor accounts for how much of that box the cut actually fills, and the specific gravity converts volume to weight. The result is an estimate with a real error band, not a substitute for weighing.
Also known as: gem weight estimator · carat weight from dimensions
Estimating what cannot be weighed
A stone set in a piece cannot go on a scale, so its weight is estimated from its measurements: length × width × depth gives a bounding box, a shape factor accounts for how much of that box the cut fills, and specific gravity converts volume to weight.
Specific gravity is why the species matters so much. Sapphire is 4.0 and emerald 2.72, so the same physical size in emerald weighs a third less. Estimating a coloured stone using diamond's numbers gives an answer that is wrong by a large margin.
The shape factor reflects how efficiently a cut fills its bounding box. An emerald cut, squarish in profile, fills more of it than a round brilliant with its tapering pavilion — 0.0025 against 0.0018.
A worked example
An oval sapphire measuring 8 × 6 × 4 mm. The calculation is 8 × 6 × 4 × 0.0020 × 4.0, which is 1.54 carats.
The same dimensions in emerald give 1.05 carats, and in amethyst 1.02. Three stones that look identical set in a ring differ by 50% in weight, which is why a valuation that guesses the species is worthless.
Where the answer misleads
Ten percent either way is a realistic error band for a well-proportioned stone, and worse for anything unusually cut. The estimate should be quoted as a range, and a valuation that presents it as a figure to two decimal places is overstating what the method can do.
Depth is the weak link. It enters the calculation linearly, and on a set stone it is the hardest dimension to measure — often it can only be estimated from the side of the setting. An error of half a millimetre in depth on a 4 mm stone is a 12% error in the answer.
Finally, this estimates weight, not value. Two sapphires of identical weight can differ by an order of magnitude in price on colour, clarity and origin, and no dimensional measurement touches any of that.
Frequently asked questions
How do I estimate the weight of a set stone?
Measure length, width and depth in millimetres, multiply them together, then multiply by the shape factor and the species' specific gravity. An 8×6×4 mm oval sapphire estimates at about 1.54 carats.
How accurate is the estimate?
Within roughly ten percent for a well-proportioned stone, and worse for anything unusually cut. Depth is the hardest measurement to take on a set stone and the one the formula is most sensitive to, since it enters linearly.
Why does the species matter?
Because specific gravity varies a lot. A sapphire is 4.0 and an emerald 2.72, so the same physical size in emerald weighs a third less. Estimating a coloured stone as if it were a diamond gives a badly wrong answer.
What is a shape factor?
An allowance for how much of the length-by-width-by-depth box the cut fills. A round brilliant fills less than an emerald cut, which is squarer in profile, so their factors differ — 0.0018 against 0.0025.