Stone Weight Estimator
Estimate a stone's carat weight from millimetre dimensions across twelve standard cuts and sixteen gem species, or work backwards from a known weight to expected dimensions. Diamonds follow the standard lab estimation formulae; coloured stones use density-based formulae with depth defaults matched to how each species is actually cut. It runs entirely in your browser and nothing is uploaded.
How it works
Pick the cut and species
Choose from twelve standard cuts and sixteen common gem species. Each species carries its own density, and the depth defaults follow how that species is actually cut, not a flat diamond assumption.
Enter the millimetres
Type the length, width, and depth from a loupe or calliper reading to build the stone's estimated volume.
Read the weight, or reverse it
See the estimated carat weight straight away, or switch to reverse mode to estimate dimensions from a known weight.
Estimation formulas assume average proportions for each cut. Real weight can swing by 10% or more depending on girdle thickness and pavilion depth, so treat this as a fast estimate, not a certificate. Weigh the stone on a scale whenever the number actually matters, such as pricing or insurance.
Want the full accurate-scale workflow guide?
This tool is free to use right now, no email needed. The guide below is optional: it walks through the anchor-measure-verify method behind True-Scale, our other free tool.
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