How accurate are the estimates?

Accuracy is typically ±5–10% for well-proportioned stones. Results vary because actual depth percentages and crown/pavilion proportions differ by cutter. The formulas use industry-standard average depth factors for each shape.

What is specific gravity and why does it matter?

Specific gravity (SG) is the ratio of a material's density to that of water. Diamond has an SG of 3.52 while emerald is only 2.72, so an emerald of the same dimensions weighs less. Selecting the correct gem type ensures the formula uses the right SG.

Why does the formula differ by cut shape?

Each cut shape has a different three-dimensional volume. A round brilliant retains a cone-like pavilion, while an emerald cut is more rectangular and step-cut with shallower proportions. Shape-specific correction factors account for these volume differences.

What if my stone's shape is not listed?

Choose the shape that most closely matches your stone. A heart shape can be approximated as a round or pear; a triangle (trillion) is close to a princess cut. The result will be an approximation.

Does this work for lab-grown gemstones?

Yes. Lab-grown diamonds, sapphires, and rubies have the same chemical composition — and therefore the same specific gravity — as their natural counterparts, so the formulas apply equally.

Gemstone Weight Calculator

Estimate the carat weight of a cut gemstone from its length, width, depth, and shape using standard gemological formulas.

Stone Measurements

Gemstone Type

Cut Shape

Length (mm)

Width (mm)

Depth (mm)

Estimated Weight

Enter measurements and click Calculate

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Summary

This tool estimates the carat weight of a cut gemstone from its measured dimensions (length, width, and depth in millimeters) and shape, using standard gemological weight estimation formulas. It supports eight common cuts and accounts for material-specific gravity so you can get accurate estimates for diamonds, sapphires, rubies, emeralds, and other popular gems.

How it works

Select the gemstone type (e.g., Diamond, Sapphire, Ruby).
Choose the cut shape from the dropdown (Round, Oval, Emerald, Cushion, Pear, Marquise, Princess, or Radiant).
Enter the length and width in millimeters — for round stones, length and width are the same diameter.
Enter the depth (height) in millimeters.
The calculator applies the shape-specific formula and the gem's specific gravity to estimate carat weight.

Use cases

Estimate the weight of a loose gemstone without a scale.
Verify a seller's stated carat weight against measured dimensions.
Compare the carat yield of different cut shapes for a given rough size.
Quickly assess gemstone value when only measurements are available.
Cross-check gemstone certificates against physical measurements.
Educate customers on how cut shape affects apparent size vs. actual weight.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu