💎 Gemstone Carat Weight Calculator
Estimate a gemstone's carat weight from its measurements, cut shape, and gem type — using each stone's real specific gravity. Plus a quick one-number estimate for round brilliants.
💎 Estimated carat weight
Quick round-brilliant estimate
Assumes a typical ~61% total depth. A 6.5 mm round brilliant diamond is the classic one-carat stone.
Estimates only — NOT a professional appraisal or investment advice. Real weight depends on the exact cut proportions, girdle thickness, and finish. For insurance, sale, or purchase, rely on a certified gemologist or appraiser weighing the stone on a calibrated scale.
Weight is density, not just size
Carat is a unit of weight, not size — one carat is exactly 0.2 grams. Two stones can look the same across the top yet weigh very differently, because a ruby is denser than an emerald and a deep cut hides weight below the girdle. That's why measuring across the face alone never tells you the carat: you also need the depth and the gem's density.
This tool takes all three dimensions, scales them by how fully each cut shape fills its bounding box, and multiplies by the gem's specific gravity. It's the same reasoning behind the factors jewellers use — reliable enough to sanity-check a listing, but never a substitute for weighing the stone.
❓ Frequently Asked Questions
How is carat weight estimated from a stone's dimensions?
Weight is physics: mass equals volume times density (specific gravity), and one carat is 0.2 grams. A faceted stone doesn't fill its bounding box, so we scale the length × width × depth by a per-shape fill factor, then multiply by the gem's specific gravity. Working in millimetres this is weight = L × W × depth × fill × SG ÷ 200.
What specific gravity does each gem use?
Real published values: diamond 3.52, ruby and sapphire (both corundum) 4.0, emerald 2.72, amethyst 2.65, topaz 3.53, garnet 3.9, tanzanite 3.35, opal 2.1, moissanite 3.22, and cubic zirconia 5.7. A denser gem weighs more at the same size — a ruby out-weighs an emerald of identical dimensions by the ratio of their densities.
Why is a 6.5 mm round diamond about one carat?
It's the classic anchor of diamond sizing. A well-cut 6.5 mm round brilliant diamond weighs almost exactly 1.00 carat, an 8 mm round is roughly 2 carats, and the quick estimate here reproduces the jewellers' round-brilliant factor (length × width × depth × 0.0061) for diamond.
How accurate is this calculator?
It's a good ballpark for well-proportioned stones, but real weight shifts with the exact cut depth, girdle thickness, bulge, and finish — a deeply cut or heavily bulged stone weighs more than its face size suggests, and a shallow one less. Treat the number as an estimate to sanity-check, not a measured weight.
Is this an appraisal?
No. These are estimates only — NOT a professional appraisal or investment advice. Carat weight is only one factor in a stone's value, and value also depends on cut, colour, clarity, treatment, and origin. For insurance, sale, or purchase, always consult a certified gemologist or appraiser who can weigh and grade the stone directly.