Stone Size Conversion Guide
A carat number tells only part of a stone's story. Millimeter measurements help you picture its outline and compare how different shapes will sit in a ring.
Weight and dimensions answer different questions
Carat measures weight: one carat is 200 milligrams. Millimeters measure dimensions. Round stones are commonly described by diameter; elongated shapes need length and width, and depth is also important for setting fit. Two stones of the same weight can look different because their shapes and proportions differ.
Approximate diamond dimensions
This small reference uses selected examples from Stuller's conversion chart. Dimensions are in millimeters; weights are approximate. It is a comparison guide, not a way to weigh a stone or confirm that it fits a mounting.
| Weight | Round diameter | Oval L × W | Emerald L × W | Pear L × W |
|---|---|---|---|---|
| 0.50 ct | 5.2 | 6.25 × 4.5 | 5.5 × 4 | 7 × 5 |
| 0.75 ct | 5.7 | 7 × 5 | 6 × 4.5 | 7.5 × 5.25 |
| 1.00 ct | 6.4 | 8 × 6 | 6.5 × 5 | 8.5 × 5.5 |
| 1.50 ct | 7.2 | 9 × 6.5 | 7.5 × 5.5 | 9.5 × 6.5 |
| 2.00 ct | 8.0 | 10 × 7 | 8.5 × 6 | 11 × 7 |
| 3.00 ct | 9.3 | 11.5 × 8 | 9.5 × 7 | 12.5 × 8 |
What about moissanite?
The table describes diamonds. Moissanite has a different density, so its actual carat weight will differ at matching dimensions. When a listing gives a diamond-equivalent weight, check the millimeter measurements to understand the comparison.
Use the individual stone's measurements
For a purchase or custom setting, use the stone's actual length, width, and depth rather than a chart estimate. Ask to compare the outline on your hand, then confirm setting compatibility. Images on phones and computers are not a reliable true-size reference because screens and zoom settings vary.