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Gem weight calculator
Enter the measurements of a mounted gem (with a gauge) and its cut shape. We estimate the weight in carats instantly — no sign-up.
The calculator interface is in Spanish; the figures are universal.
Diámetro² × altura × P.E. × 0,0018
Selecciona una gema e introduce las medidas para ver el peso estimado.
Estimación orientativa a partir de fórmulas gemológicas estándar (peso específico × volumen × coeficiente de talla). Válida para piedras montadas cuando no se puede pesar directamente. No sustituye una certificación gemológica.
How the calculation works
We multiply the approximate volume of the stone (based on its cut shape) by the specific gravity of the mineral — a physical constant published in standard gemological manuals (GIA, Webster).
An indicative estimate for stones that cannot be weighed directly because they are mounted. It does not replace a gemological certification or a direct weighing.
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Start 14-day trial →How the weight of a mounted stone is calculated
A set stone cannot be placed on the scale without unmounting it — and unmounting carries risk and cost. The appraiser’s alternative is to estimate the weight from the measurements: the volume of the stone is approximated from its cut and multiplied by the specific gravity of the mineral, the density that turns cubic millimeters into carats.
For a round brilliant cut, the formula is diameter² × height × SG × 0.0018. You may know the classic diamond formula (diameter² × height × 0.0061): it is the same, with the specific gravity of diamond (3.52) already baked into the coefficient. The general version works for any gem — which is why this calculator asks for the stone type first.
Specific gravity of the most common gems
The same volume weighs very differently depending on the mineral: a 6 mm sapphire weighs considerably more than a diamond of the same size. That is why two stones that look “the same” are not equivalent in carats.
| Gem | Specific gravity (g/cm³) |
|---|---|
| Sapphire | 4.00 |
| Ruby | 4.00 |
| Rhodolite garnet | 3.84 |
| Topaz | 3.53 |
| Diamond | 3.52 |
| Peridot | 3.34 |
| Tourmaline | 3.06 |
| Emerald | 2.72 |
| Aquamarine | 2.69 |
| Amethyst (quartz) | 2.65 |
Reference values from standard gemological manuals; some varieties show slightly different ranges. The calculator applies the specific gravity of each of its 100+ gem types.
The limits of volume-based calculation
The coefficients assume standard cut proportions. A very deep or very shallow pavilion, a thick girdle or an old cut push the result away from the real weight — typical deviations are around ±10%, and larger in fancy cuts. That is why the result is an estimate, and it must be declared as such in any document: in ValorGem reports, a weight computed from measurements is automatically marked as estimated.
For a singular stone or a doubtful identification, the estimate does not replace a professional’s judgment. You can consult our gemologists for a second opinion on the piece.