Round brilliant: carat to diameter
These are approximate, industry-standard figures for a well-cut round brilliant. They depend on cut proportions. Two stones of the same weight can differ by a few tenths of a millimetre. The report states the exact measurements.
| Carat | Diameter (mm) |
|---|---|
| 0.005 | 1.0 |
| 0.01 | 1.3 |
| 0.02 | 1.7 |
| 0.03 | 2.0 |
| 0.04 | 2.2 |
| 0.05 | 2.4 |
| 0.10 | 3.0 |
| 0.15 | 3.4 |
| 0.20 | 3.8 |
| 0.25 | 4.1 |
| 0.30 | 4.3 |
| 0.40 | 4.8 |
| 0.50 | 5.1 |
| 0.60 | 5.4 |
| 0.70 | 5.7 |
| 0.75 | 5.8 |
| 0.90 | 6.2 |
| 1.00 | 6.5 |
| 1.25 | 6.9 |
| 1.50 | 7.4 |
| 1.75 | 7.8 |
| 2.00 | 8.1 |
| 2.50 | 8.8 |
| 3.00 | 9.3 |
| 4.00 | 10.2 |
| 5.00 | 11.0 |
Melee sizes
Melee means small diamonds, generally under 0.20 carat, sold by the parcel. At this size, a tenth of a millimetre changes the weight a lot, so buyers usually specify diameter and total carat weight together.
The rows from 0.005 to 0.20 carat above are the usual melee range. Melee is graded in-house and sold without a lab report. See melee & parcels.
Fancy shapes: approximate face-up size
Fancy shapes hide their weight differently. A deep stone looks small for its carat, and a shallow one looks large. Sizes are length x width in millimetres, or one side for square shapes. Approximate only.
| Shape | 0.5 ct | 1 ct | 1.5 ct | 2 ct |
|---|---|---|---|---|
| Oval | 6.0 x 4.0 | 7.7 x 5.5 | 8.5 x 6.0 | 9.0 x 6.5 |
| Cushion | 4.5 | 5.5 | 6.3 | 6.9 |
| Emerald | 5.5 x 3.8 | 6.5 x 4.5 | 7.5 x 5.5 | 8.0 x 6.0 |
| Pear | 6.5 x 4.0 | 8.0 x 5.0 | 9.5 x 6.0 | 10.5 x 6.5 |
| Princess | 4.0 | 5.5 | 6.2 | 6.8 |
Use the report, not the chart
A chart is a guide for planning. The lab report gives the exact measurements for each certified stone. Check the report on the lab's own site: how to verify a diamond certificate.
Need a stone to a set size? Ask us to source it.
Questions
How big is a 1 carat round diamond in mm?
About 6.5 mm across for a well-cut round brilliant. Cut proportions change this slightly, so check the report.
Why do two stones of the same carat look different in size?
Carat is weight. A stone cut deep carries weight in its depth and looks smaller face-up. A shallower cut spreads the weight and looks larger.
Are these sizes exact?
No. They are approximate and depend on cut proportions. The grading report states the exact measurements for a certified stone.
What is melee in mm?
Melee is generally under 0.20 carat, which is under about 3.8 mm in a round. It is sold by the parcel and graded in-house. See melee & parcels.
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