Diamond Weight Estimator
Estimate diamond carat weight from length, width, depth, and cut shape.
Returns weight estimate for round, princess, oval, pear, and emerald cut diamonds.
Diamond carat weight can be estimated from physical measurements using shape-specific formulas. This is useful when a diamond is already set in jewelry and cannot be weighed directly on a scale.
General Formula:
Carat Weight = Length × Width × Depth × Shape Factor
All measurements are in millimeters (mm). The shape factor accounts for the different volume distributions of each diamond cut.
Shape Factors:
| Diamond Shape | Shape Factor | Formula Notes |
|---|---|---|
| Round Brilliant | 0.0061 | Most common cut; uses average diameter |
| Princess (Square) | 0.0083 | Square shape; L and W are similar |
| Oval | 0.0062 | Elongated shape |
| Emerald | 0.0080 | Rectangular step-cut |
| Pear | 0.0059 | Teardrop shape |
| Marquise | 0.0059 | Boat/eye shape |
| Cushion | 0.0074 | Rounded square |
| Radiant | 0.0081 | Trimmed-corner rectangle |
| Asscher | 0.0080 | Square step-cut |
| Heart | 0.0059 | Heart shape |
Worked Example — Round Diamond: A round diamond measures 6.5 mm diameter × 4.0 mm depth: Weight = 6.5 × 6.5 × 4.0 × 0.0061 = 1.031 carats
Worked Example — Princess Cut: A princess diamond measures 5.5 mm × 5.5 mm × 3.8 mm: Weight = 5.5 × 5.5 × 3.8 × 0.0083 = 0.954 carats
Worked Example — Oval: An oval diamond measures 8.0 mm × 5.5 mm × 3.5 mm: Weight = 8.0 × 5.5 × 3.5 × 0.0062 = 0.955 carats
Typical dimensions by carat weight (Round Brilliant):
| Carats | Diameter (mm) | Depth (mm) |
|---|---|---|
| 0.25 | 4.1 | 2.5 |
| 0.50 | 5.2 | 3.1 |
| 0.75 | 5.8 | 3.5 |
| 1.00 | 6.5 | 3.9 |
| 1.25 | 6.9 | 4.2 |
| 1.50 | 7.4 | 4.5 |
| 2.00 | 8.2 | 5.0 |
| 3.00 | 9.4 | 5.7 |
Important notes:
- These are estimates. Actual weight can vary by 5–10% depending on the exact proportions and cut quality.
- Deeper-cut diamonds will weigh more for the same face-up diameter.
- Poorly proportioned diamonds may appear smaller than their carat weight suggests.
- For accurate carat weight, the diamond should be weighed on a gemological scale (0.01 ct precision).
How to measure a mounted diamond:
- Use a jewelry caliper or millimeter gauge.
- For round diamonds, measure the diameter from two directions and average them.
- Depth can be measured with a leveridge gauge or estimated from the side profile.
- If the diamond is in a bezel setting, only approximate measurements may be possible.
Understanding carats:
- 1 carat = 200 milligrams (0.2 grams)
- The word “carat” comes from the carob seed, which was historically used as a balance weight.
- Carat weight is one of the “4 Cs” of diamond grading (Cut, Color, Clarity, Carat).