Gemstone Weight Calculator
Estimate gemstone carat weight from stone measurements, cut shape, and gemstone type. Designed for jewelers, CAD designers, and anyone planning a custom jewelry setting.
Configure your gemstone
Select the cut and gemstone type, then enter the stone measurements.
Specific gravity (SG) varies by gem variety and composition. Select “custom” if you know the actual SG.
Measure maximum face-up length and width, plus total stone depth (table to culet).
Thicker + / thinner −
Bulging pavilion +
How gemstone carat weight is estimated
For typical faceted gemstones, a practical approximation is estimated carats = length × width × depth × specific gravity × shape factor. Shape factors account for the difference between a rectangular volume and the cut gemstone. The round calculation uses the average of two face-up diameters. These are estimation formulas, not measured weights.
What is a carat?
One carat (ct) equals 0.20 grams, or 200 milligrams. One carat is also 100 points, so a 0.50 ct gemstone weighs 50 points.
What do points in a carat mean?
In the jewelry industry, points are a unit of gemstone weight: 100 points equal exactly 1 carat. One point is 0.01 carat, or 2 milligrams. For example, a 50-point diamond weighs 0.50 carat.
Quick Conversion Reference
| Points | Carats (ct) | Fraction of a carat |
|---|---|---|
| 10 pt | 0.10 ct | One-tenth |
| 25 pt | 0.25 ct | One-quarter |
| 50 pt | 0.50 ct | One-half |
| 75 pt | 0.75 ct | Three-quarters |
| 100 pt | 1.00 ct | One full carat |
Why do two gemstones of the same size weigh differently?
Gem species have different specific gravities. Shape, girdle thickness, pavilion contour, and cut proportions also affect the amount of material in a stone of the same face-up dimensions.
How should I measure a gemstone?
Use a calibrated digital caliper. Record maximum face-up length and width, then measure total depth from the table to the culet or lowest point. For a round stone, measure two perpendicular diameters. Mounted stones can be difficult to measure accurately.
Can I use estimated carat weight to design a setting?
Carat weight alone is not enough for CAD or setting design. Use the stone’s actual length, width, depth, girdle profile, and setting tolerances. The stone should be checked directly before final manufacturing.
Methodology reference: GIA — Estimating Weights of Mounted Colored Stones; International Gem Society — Estimating Gem Weight. The factors below are simplified practical approximations; no calculator can replace direct weighing.