mm to carat calculator: diamond and gemstone weight from size
By HeyGema™ · Updated
In short
A round diamond of 6.5 mm weighs about 1 carat, 8 mm about 1.91 carat and 10 mm about 3.73 carat. Weight grows with the cube of the diameter, so a few tenths of a millimeter change the carats a lot.
Carat to mm calculator
About 1.02 carat
An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.
How to convert mm to carat
For a well-cut round diamond, the weight in carats is about (diameter / 6.45)³. A 7 mm round is (7 / 6.45)³ = 1.28 carat. For other shapes use the length and width, and for other gems multiply by the gem's density over diamond's (3.52). If you know the depth too, the diamond weight calculator uses all three measurements.
mm to carat chart for round diamonds
- 2 mm
- Diamond: 0.030 ct
- Moissanite (true weight): 0.030 ct
- Sapphire: 0.030 ct
- 3 mm
- Diamond: 0.1 ct
- Moissanite (true weight): 0.090 ct
- Sapphire: 0.11 ct
- 4 mm
- Diamond: 0.24 ct
- Moissanite (true weight): 0.22 ct
- Sapphire: 0.27 ct
- 5 mm
- Diamond: 0.47 ct
- Moissanite (true weight): 0.43 ct
- Sapphire: 0.53 ct
- 5.5 mm
- Diamond: 0.62 ct
- Moissanite (true weight): 0.57 ct
- Sapphire: 0.7 ct
- 6 mm
- Diamond: 0.8 ct
- Moissanite (true weight): 0.74 ct
- Sapphire: 0.91 ct
- 6.5 mm
- Diamond: 1.02 ct
- Moissanite (true weight): 0.94 ct
- Sapphire: 1.16 ct
- 7 mm
- Diamond: 1.28 ct
- Moissanite (true weight): 1.17 ct
- Sapphire: 1.45 ct
- 7.5 mm
- Diamond: 1.57 ct
- Moissanite (true weight): 1.44 ct
- Sapphire: 1.79 ct
- 8 mm
- Diamond: 1.91 ct
- Moissanite (true weight): 1.75 ct
- Sapphire: 2.17 ct
- 9 mm
- Diamond: 2.72 ct
- Moissanite (true weight): 2.49 ct
- Sapphire: 3.09 ct
- 10 mm
- Diamond: 3.73 ct
- Moissanite (true weight): 3.41 ct
- Sapphire: 4.23 ct
- 11 mm
- Diamond: 4.96 ct
- Moissanite (true weight): 4.54 ct
- Sapphire: 5.64 ct
- 12 mm
- Diamond: 6.44 ct
- Moissanite (true weight): 5.89 ct
- Sapphire: 7.32 ct
- 15 mm
- Diamond: 12.58 ct
- Moissanite (true weight): 11.51 ct
- Sapphire: 14.29 ct
Why a small size change moves the weight
Weight follows volume, which follows the cube of the diameter. Going from 6.5 mm to 7 mm is 8% wider but 25% heavier. That is why listings in mm are precise and weights alone are not. See the melee chart for small stones.