Gem Weight Calculator

Estimate a gemstone’s carat weight from measurements and specific gravity (SG). Use for quick appraisals and educational purposes.

Stone shape Generic (L×W×D) Round / Brilliant (Diameter × Depth) Oval / Marquise (L×W×D) Emerald / Step cut (L×W×D, use actual depth)

Units Millimetres (mm) Centimetres (cm)

Length

Width (or Diameter)

Depth

Specific Gravity (SG) Diamond — 3.52 Corundum (Sapphire/Ruby) — 4.00 Quartz (Amethyst) — 2.65 Beryl (Emerald) — 2.68 Topaz — 3.53 Custom SG

Custom Specific Gravity

Calculate Carat Reset

Result:

Enter stone dimensions and SG, then click Calculate Carat.

Formula & notes

Core shortcut formula for faceted gems:

Carat = (Length × Width × Depth × SG) ÷ 200

Measurements in mm. For round stones set Length = Width = Diameter. This method gives an estimate — for high-value gems always confirm by precise weighing.

How the Gem Weight Calculator Works

A Gem Weight Calculator estimates a gemstone’s carat weight using its measured dimensions and the gem’s specific gravity (SG). The tool converts the stone’s approximate volume into mass (grams) and then converts that mass into carats. This method is a practical alternative when a precision scale isn’t available, and it’s widely used by hobbyists, jewelers, and appraisers for quick estimates.

Core Formula

For most faceted stones the shortcut formula is:

Carat = (Length × Width × Depth × Specific Gravity) ÷ 200

• Length, Width and Depth are in millimetres (mm).
• Specific Gravity (SG) is unitless (density relative to water).
• Dividing by 200 accounts for unit conversions (mm³ → cm³ and grams → carats; 1 carat = 0.2 g).

For round/brilliant cuts use Diameter for length and width (Length = Width = Diameter). For irregular shapes the formula becomes less accurate; consider weighing instead.

Worked Example

A sapphire (SG ≈ 4.00) measures 7.0 mm × 5.0 mm × 3.5 mm.

1. Calculate volume: 7.0 × 5.0 = 35.0 → 35.0 × 3.5 = 122.5 mm³.
2. Multiply by SG: 122.5 × 4.00 = 490.0.
3. Divide by 200: 490.0 ÷ 200 = 2.45 carats.

So, the estimated carat weight ≈ 2.45 carats. This matches the step-by-step volume → mass → carat conversion.

Common Specific Gravities (Approx.)

• Diamond: 3.52
• Sapphire / Ruby (Corundum): 4.00
• Quartz (Amethyst/Citrine): 2.65
• Emerald / Beryl: 2.68
• Topaz: 3.53

Use the correct SG for the gem species to improve accuracy.

Tips for Better Accuracy

• Use digital calipers for measurements and take the widest points.
• Measure depth precisely—depth errors have large effects.
• For cabochons or carved stones, the formula is less reliable.
• Repeat measurements and average the results.
• For valuable stones, always verify with a calibrated scale and gem lab report.

Final Note

This calculator provides a fast, reliable estimate useful for valuation, trading, and education. For certification or high-value transactions, rely on professional weighing and gemological testing.