Carat weight vs millimetre size: why jewellery CAD uses mm
A carat measures a stone's weight, not its size. A CAD model, on the other hand, needs the stone's dimensions in millimetres. The two are connected for a given stone shape, but they are not the same thing.
Approximate round diamond sizes
| Weight | Approximate diameter |
|---|---|
| 0.25 ct | about 4.1 mm |
| 0.50 ct | about 5.1 mm |
| 0.75 ct | about 5.8 mm |
| 1.00 ct | about 6.5 mm |
| 1.50 ct | about 7.4 mm |
| 2.00 ct | about 8.1 mm |
Why the numbers vary
Two stones of the same weight can look slightly different in size, depending on the cut proportions. Other shapes, such as oval and marquise, have different length and width for the same weight.
What to tell your designer
- The exact measurements of the stone, if you have it
- If not, the carat weight, the shape and the cut quality
The designer can then build the seat to the right size. Put these details in your design brief.
Frequently asked questions
Is a 1 carat diamond always 6.5 mm?
No. This is an average for a well-cut round stone. Real stones vary a little.
Do lab-grown and natural diamonds of the same carat have the same size?
Yes. Size depends on the weight and the cut, not on how the stone formed.
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