A 3 carat round diamond measures about 9.28 mm across and covers roughly 67.6 mm² seen from above.
Carat is weight, not size, so the same 3 ct buys visibly different dimensions depending on shape. At this weight the gap between the shape that spreads widest (emerald) and the most compact (cushion) is about 24% of face-up area — for the same weight, and broadly the same price.
| Shape | Length × width (mm) | Face-up area (mm²) |
|---|---|---|
| Round | 9.28 × 9.28 | 67.6 |
| Oval | 11.41 × 8.15 | 73.1 |
| Cushion | 8.42 × 8.02 | 60.4 |
| Pear | 12.17 × 7.85 | 65.0 |
| Marquise | 14.08 × 7.41 | 73.3 |
| Emerald | 10.42 × 7.44 | 75.0 |
| Radiant | 9.22 × 7.69 | 68.2 |
| Princess | 7.91 × 7.91 | 62.6 |
| Asscher | 8.20 × 8.20 | 64.2 |
Face-up area is what the eye reads as "big". Two 3 ct stones can differ by a fifth in the area they cover while weighing exactly the same, because depth hides weight underneath, where nobody sees it.
A figure in millimetres is hard to picture, and photographs of other people's hands are worse than useless — the same stone on a slender finger looks like a bigger stone. This tool renders it at true millimetre scale on a photo of your hand, calibrated from your ring size or a bank card, so the size on screen is the real one. The photo is processed on your device and never uploaded.
See 3 ct on your own hand →