Estimate Diamond Carat Weight by Measurements
Enter dimensions in millimeters (mm). This tool gives an estimated carat weight based on standard shape formulas.
How this diamond weight calculator works
Diamond carat is technically a unit of mass, not visible size. But in the jewelry industry, gemologists often estimate carat weight from physical dimensions when a stone is mounted or when scale access is limited. This calculator uses common industry approximation factors tied to each shape.
For round diamonds, the formula is: diameter × diameter × depth × shape factor.
For fancy shapes (oval, emerald, pear, etc.), the formula is: length × width × depth × shape factor.
Shape factors used in this tool
| Shape | Factor |
|---|---|
| Round Brilliant | 0.0061 |
| Princess | 0.0080 |
| Emerald | 0.0092 |
| Asscher | 0.0090 |
| Oval | 0.0062 |
| Radiant | 0.0080 |
| Pear | 0.0059 |
| Marquise | 0.0057 |
| Heart | 0.0060 |
| Cushion | 0.0090 |
When to use a carat estimate
Mounted diamonds
If a stone is already set in a ring or pendant, removing it to weigh may be impractical. A dimension-based estimate helps jewelers and buyers make informed decisions quickly.
Pre-purchase comparisons
If you are comparing listings and only millimeter dimensions are available, this calculator can help you spot stones that may be shallow or deep relative to expected weight.
Parcel planning
For matching sets (earrings, side stones, halo layouts), estimated weight per stone and total parcel weight can support budgeting and design planning.
Tips for better accuracy
- Measure with a digital caliper to at least 0.01 mm precision.
- Take multiple readings and average them before calculating.
- Use the exact shape factor that best matches the stone outline.
- Remember that cut quality and girdle thickness can shift true carat weight.
- Confirm important valuations with a certified gemologist or scale measurement.
Frequently asked questions
Is this calculator exact?
No. It is an estimate. Real weight may differ by a few percent or more depending on proportions.
What units should I enter?
Use millimeters for diameter, length, width, and depth.
Does this work for lab-grown diamonds?
Yes. The geometric estimate is based on dimensions and shape, so it applies to both natural and lab-grown stones.