Carat to mm calculator: diamond and gemstone size from weight
By HeyGema™ · Updated
In short
Carat is weight and mm is length, so the answer depends on the shape and the gem. A round diamond is about 6.45 mm across at 1 carat, 8.1 mm at 2 carat and 11.0 mm at 5 carat. Use the calculator for any shape or gem.
Carat to mm calculator
About 6.5 x 6.5 mm
An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.
How to convert carat to mm
For a well-cut round diamond, the diameter in mm is about 6.45 times the cube root of the carat weight. Other shapes use their own 1 carat size, and other gems scale by density: each side is multiplied by the cube root of 3.52 divided by the gem's specific gravity (3.52 is diamond's). The methodology has every number.
Example: a 3 carat round diamond is 6.45 x the cube root of 3 (1.442) = 9.3 mm.
Carat to mm chart for round diamonds
- 0.1 ct
- Diameter: 2.99 mm
- Inches: 0.12 in
- 0.25 ct
- Diameter: 4.1 mm
- Inches: 0.16 in
- 0.5 ct
- Diameter: 5.1 mm
- Inches: 0.20 in
- 0.75 ct
- Diameter: 5.9 mm
- Inches: 0.23 in
- 1 ct
- Diameter: 6.5 mm
- Inches: 0.25 in
- 1.5 ct
- Diameter: 7.4 mm
- Inches: 0.29 in
- 2 ct
- Diameter: 8.1 mm
- Inches: 0.32 in
- 3 ct
- Diameter: 9.3 mm
- Inches: 0.37 in
- 4 ct
- Diameter: 10.2 mm
- Inches: 0.40 in
- 5 ct
- Diameter: 11.0 mm
- Inches: 0.43 in
- 7 ct
- Diameter: 12.3 mm
- Inches: 0.49 in
- 10 ct
- Diameter: 13.9 mm
- Inches: 0.55 in
- 20 ct
- Diameter: 17.5 mm
- Inches: 0.69 in
- 50 ct
- Diameter: 23.8 mm
- Inches: 0.94 in
Other shapes and gems
Pick the shape in the calculator, or see the chart for all 12 shapes and the gemstone size chart. For the reverse, use the mm to carat calculator.