Diamond size chart
A 1 carat round diamond is about 6.5 mm across and a 2 carat about 8 mm on the IGS chart. Calibrate your screen once to see every size at real size.
Diamond size chart: the tool
Carat to mm
Approximate for a well-cut round brilliant. A shallow cut looks bigger, a deep one smaller, at the same weight.
Actual size on screen
Carat size chart: round diamond mm to carat
| Diameter | Carat |
|---|---|
| 1.6 mm | 0.02 ct |
| 2 mm | 0.03 ct |
| 2.5 mm | 0.06 ct |
| 3 mm | 0.1 ct |
| 3.5 mm | 0.16 ct |
| 4 mm | 0.25 ct |
| 4.5 mm | 0.33 ct |
| 5 mm | 0.5 ct |
| 5.5 mm | 0.65 ct |
| 6 mm | 0.75 ct |
| 6.5 mm | 1 ct |
| 7 mm | 1.25 ct |
| 8 mm | 2 ct |
| 9 mm | 2.75 ct |
| 10 mm | 3.5 ct |
| 11 mm | 4.6 ct |
| 12 mm | 6 ct |
| 13 mm | 7.75 ct |
| 14 mm | 9.65 ct |
| 15 mm | 11.85 ct |
| 16 mm | 14.35 ct |
| 17 mm | 17.25 ct |
| 18 mm | 20.5 ct |
| 19 mm | 24 ct |
| 20 mm | 28 ct |
| 21 mm | 34.8 ct |
| 22 mm | 41.8 ct |
| 23 mm | 49.8 ct |
| 24 mm | 57.9 ct |
| 25 mm | 68.7 ct |
Square (princess) diamond
| Side | Carat |
|---|---|
| 5 mm | 0.75 ct |
| 6 mm | 1.3 ct |
| 7 mm | 2.05 ct |
| 8 mm | 2.9 ct |
| 9 mm | 4.4 ct |
| 10 mm | 6 ct |
| 11 mm | 8 ct |
| 12 mm | 10.35 ct |
| 13 mm | 13.2 ct |
| 14 mm | 16.5 ct |
| 15 mm | 20.25 ct |
| 16 mm | 24.6 ct |
Oval diamond
| Length x width | Carat |
|---|---|
| 5 x 3 mm | 0.25 ct |
| 6 x 4 mm | 0.5 ct |
| 7 x 5 mm | 1 ct |
| 8 x 6 mm | 1.5 ct |
| 9 x 7 mm | 2.5 ct |
| 10 x 8 mm | 3 ct |
| 11 x 9 mm | 4 ct |
| 12 x 10 mm | 5 ct |
| 14 x 10 mm | 6 ct |
| 14 x 12 mm | 8 ct |
| 16 x 12 mm | 10.5 ct |
| 18 x 13 mm | 14.2 ct |
| 20 x 15 mm | 20.3 ct |
| 22 x 15 mm | 23.5 ct |
| 23 x 17 mm | 32.2 ct |
| 25 x 18 mm | 43 ct |
| 27 x 20 mm | 58.5 ct |
| 30 x 22 mm | 79.6 ct |
Carat is weight, not size
One carat is 0.2 g. Two stones of the same weight can measure differently depending on how deep they are cut, so these sizes are typical, not exact. The formulas gem labs use multiply length, width and depth by a factor for each shape (for a round brilliant, about 0.0061).
Related tools
Questions
How big is a 1 carat diamond in mm?
About 6.5 mm across for a round brilliant on the IGS chart.
How big is a 2 carat diamond?
About 8 mm across for a round brilliant.
Does a 2 carat diamond look twice as big as 1 carat?
No. Weight grows with volume, so the width grows much less: 6.5 mm at 1 ct against 8 mm at 2 ct on this chart.
Sources
- International Gem Society: Standard Gem Sizes Chart: Guide to Gemstone Dimensions (checked 2026-10-02)
- GCAL (Gem Certification & Assurance Lab): Instructional Diamond Grading (weight estimation formulas) (checked 2026-10-02)
Last checked: 5 October 2026