Estimate gemstone carat weight from dimensions and shape. Supports round, oval, pear, emerald, cushion, marquise, and princess cuts with density adjustments.
Estimating a gemstone's carat weight from its physical dimensions is essential for jewelers, gemologists, and buyers who need to verify or approximate weight without removing a stone from its setting. Each cut shape has a specific mathematical relationship between length, width, depth, and weight, so a quick size check can tell you whether a stone is plausible before it ever reaches a scale.
This carat weight calculator uses industry-standard estimation formulas that account for the shape factor of each cut. A round brilliant has a different volume-to-weight conversion than an oval or emerald cut, and the calculator applies the correct coefficients for each. The results are typically accurate within 5–10% of actual weigh-in values when the stone follows normal proportions.
The tool also adjusts for gemstone species, since different minerals have different densities. A sapphire of the same dimensions as a diamond will weigh more (specific gravity ~4.0 vs ~3.52). By selecting the gem type, you get a density-corrected estimate that is more useful for pricing, appraisal, and comparison.
Whether you're appraising estate jewelry where stones cannot be removed, verifying a seller's claim, or learning how shape and density affect weight, this calculator gives a fast estimate from simple measurements.
Use this calculator when you need a weight estimate from dimensions because a stone is mounted, sealed in a setting, or otherwise cannot be weighed directly. It is useful for appraisal work, shopping comparisons, and checking whether a stated gemstone weight is plausible for the measured size, cut, and species before you rely on it.
Round: Weight = Diameter² × Depth × 0.0061. Oval: Weight = Length × Width × Depth × 0.0062. Emerald: Weight = L × W × D × 0.0083. Pear: Weight = L × W × D × 0.0059. Marquise: Weight = L × W × D × 0.0059. All formulas assume diamond density (3.52 g/cm³); multiply by (gem_SG / 3.52) for other stones.
Result: 1.03 carats
A round diamond measuring 6.5mm diameter × 4.0mm depth estimates to approximately 1.03 carats using the standard round brilliant formula.
Different gemstone species have different specific gravities, which directly affects weight for a given size. **Diamond:** 3.52, **Ruby/Sapphire:** 3.97–4.05, **Emerald:** 2.67–2.78, **Aquamarine:** 2.68–2.74, **Topaz:** 3.49–3.57, **Amethyst:** 2.65, **Tanzanite:** 3.35, **Garnet:** 3.5–4.3, **Tourmaline:** 3.02–3.26, **Opal:** 1.98–2.25.
This means a ruby the same size as a diamond weighs about 14% more, while an emerald weighs about 24% less. Always select the correct gem type for accurate weight estimation.
Each cut shape has a specific mathematical factor that converts dimensions to approximate weight. Round brilliants use the simplest formula (diameter squared times depth times 0.0061). Other shapes require length × width × depth multiplied by a shape-specific constant. These constants were derived empirically from thousands of weighed stones.
Jewelers use dimensional weight estimation when appraising estate jewelry where removing stones risks damage. Insurance appraisers rely on it when documenting jewelry for replacement value. Buyers use it to verify seller claims—if a stone is advertised as 1.50 carats but measures only 6.8mm diameter, the claimed weight is likely overstated for a well-cut round diamond.
Typically within 5–10% of actual weight. Accuracy depends on measurement precision, stone proportion, and how closely the gem matches the standard cut assumptions used by the formula.
Different minerals have different densities (specific gravity). Sapphire is denser (4.0 vs 3.52), so a sapphire of the same dimensions weighs about 14% more than a diamond.
A digital caliper or gem gauge that reads to 0.01mm is ideal. A Leveridge gauge is the standard jeweler's tool for measuring set stones.
These formulas assume standard proportions. Extremely deep or shallow cuts will show more estimation error. Adjust depth manually if the stone is unusual.
One carat = 0.2 grams. Size depends on cut and density—a 1-carat round diamond is about 6.5mm, but a 1-carat round sapphire is about 6.0mm due to higher density.
Yes. Select the closest standard shape. For hearts, use the pear formula. For trillion/trilliant, use the emerald formula as an approximation.