Gemstone Carat Weight Calculator
Estimate the carat weight of a mounted or loose gemstone from its length, width and depth using shape factors and the stone’s specific gravity.
How it works
Weight (ct) = area proxy × depth × shape factor × (SG ÷ 3.52) · area proxy = L×W (or average-diameter² for round) · 1 ct = 0.2 g
You cannot weigh a set stone, so gemologists estimate it from measurements. The classic formula multiplies the girdle area by depth and a shape-specific factor calibrated to diamond (SG 3.52). Scaling by the stone’s own specific gravity extends it to other materials — a sapphire (SG ~4.0) of the same size weighs about 14% more than a diamond. Real girdle thickness and cut vary, so treat the result as a close estimate, not a certificate weight.
Worked example
A 6.5 mm round with 4 mm depth: average diameter 6.5, so area proxy 6.5² = 42.25. Weight = 42.25 × 4 × 0.0061 × (3.52/3.52) ≈ 1.03 ct. In grams that is about 0.206 g.
Frequently asked questions
How accurate is a measured estimate?
Usually within about 10% for a well-cut stone. Thick girdles, bulging pavilions or unusual cutting can push the real weight higher than the formula predicts.
Why does specific gravity matter?
Two stones of identical dimensions weigh differently if their density differs. The base factors assume diamond; multiplying by SG/3.52 corrects for sapphire, emerald, quartz and other materials.
What is a carat exactly?
One metric carat is 0.2 grams (200 milligrams), divided into 100 points. A "50-point" stone is 0.50 ct.
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This calculator is for educational purposes. Double-check important results before acting on them.