Carat size calculator
Turn any carat weight into millimetres and face-up area for ten shapes, drawn to scale next to a 1 carat round.
- Length
- 6.44mm
- Width
- 6.44mm
- Face-up area
- 32.5mm²
Your stone1.00 ct roundRuler and grid in millimetres, true relative scale
A 1.00 ct round at 61.5% depth. Depth: 3.96 mm.
Carat to millimetres, at a glance
Typical length × width for six popular shapes. Round and princess show a single measurement because they are symmetrical.
Swipe the table to see every column
| Weight | Round | Oval | Cushion | Princess | Emerald | Radiant |
|---|---|---|---|---|---|---|
| 0.25 ct | 4.1 | 5.0 × 3.7 | 3.8 × 3.5 | 3.5 | 4.5 × 3.1 | 4.2 × 3.4 |
| 0.50 ct | 5.1 | 6.3 × 4.6 | 4.8 × 4.4 | 4.4 | 5.6 × 3.9 | 5.3 × 4.2 |
| 0.75 ct | 5.9 | 7.2 × 5.3 | 5.5 × 5.0 | 5.1 | 6.4 × 4.4 | 6.1 × 4.9 |
| 0.90 ct | 6.2 | 7.6 × 5.6 | 5.9 × 5.4 | 5.4 | 6.8 × 4.7 | 6.4 × 5.2 |
| 1.00 ct | 6.4 | 7.9 × 5.8 | 6.1 × 5.5 | 5.6 | 7.1 × 4.9 | 6.7 × 5.3 |
| 1.25 ct | 6.9 | 8.5 × 6.3 | 6.6 × 6.0 | 6.0 | 7.6 × 5.2 | 7.2 × 5.8 |
| 1.50 ct | 7.4 | 9.0 × 6.7 | 7.0 × 6.3 | 6.4 | 8.1 × 5.6 | 7.6 × 6.1 |
| 2.00 ct | 8.1 | 9.9 × 7.4 | 7.7 × 7.0 | 7.0 | 8.9 × 6.1 | 8.4 × 6.7 |
| 2.50 ct | 8.7 | 10.7 × 7.9 | 8.3 × 7.5 | 7.6 | 9.6 × 6.6 | 9.1 × 7.2 |
| 3.00 ct | 9.3 | 11.4 × 8.4 | 8.8 × 8.0 | 8.0 | 10.2 × 7.0 | 9.6 × 7.7 |
| 4.00 ct | 10.2 | 12.5 × 9.3 | 9.7 × 8.8 | 8.8 | 11.2 × 7.7 | 10.6 × 8.5 |
| 5.00 ct | 11.0 | 13.5 × 10.0 | 10.4 × 9.5 | 9.5 | 12.1 × 8.3 | 11.4 × 9.1 |
How we calculate it
Gemologists estimate a mounted diamond's weight from its measurements with a simple formula: length × width × depth × shape factor. We run the same formula backwards. Given your carat weight, a typical length-to-width ratio and depth for the shape, we solve for width, then length and depth.
| Shape | L/W ratio | Depth | Factor |
|---|---|---|---|
| Round | 1.00 | 61.5% | 0.0061 |
| Oval | 1.35 | 60.0% | 0.0062 |
| Cushion | 1.10 | 66.0% | 0.0081 |
| Princess | 1.00 | 70.0% | 0.0083 |
| Emerald | 1.45 | 65.0% | 0.0092 |
| Radiant | 1.25 | 65.0% | 0.0081 |
| Pear | 1.55 | 60.0% | 0.0060 |
| Marquise | 2.00 | 58.0% | 0.0058 |
| Asscher | 1.00 | 66.0% | 0.0080 |
| Heart | 1.00 | 60.0% | 0.0059 |
Reading the result
- Diameter beats carat. Going from 1.00 to 1.50 ct in a round adds about 0.9 mm of diameter, not 50% more visible size.
- Elongated shapes spread. At 1 ct an oval covers about 36 mm² against 33 mm² for a round.
- Finger size matters. The same stone looks larger on a slimmer finger and with a thin band.
Questions readers ask
How many millimetres is a 1 carat diamond?
A well-cut 1.00 ct round brilliant measures about 6.4 mm across. Other shapes differ: a 1 ct oval is roughly 7.9 × 5.8 mm.
Why do two 1 carat diamonds look different in size?
Carat is weight. A deep stone hides weight below the girdle and faces up smaller; a shallow one spreads wider but can leak light. Compare diameter and face-up area, not just carats.
Which shape looks biggest for its weight?
Elongated shapes such as marquise, oval and pear spread their weight over a larger face-up area than round, princess or asscher cuts of the same weight.
How accurate is this calculator?
It uses the standard gem weight-estimation formula with typical proportions for each shape. Real stones vary with depth, girdle thickness and ratio, usually by a few tenths of a millimetre. The grading report’s measurements are the final word.
Is 0.90 ct really almost the same as 1.00 ct?
For rounds, yes: about 6.21 mm versus 6.44 mm across, a difference that is hard to see once the stone is set.