Round Diamond Carat Size Chart

We’ve estimated the average diameter (mm) for excellent cut round diamonds using a sample of diamonds listed for sale. Many diamond size charts tend to overestimate the diameter of a diamond for its carat weight. The diameters below are based on actual diamonds listed for sale rather a theoretical carat/mm conversion formula.

Relative sizes for each carat weight are illustrated in the figure below the table.

Average Diameter of Round Diamonds
Carat	Diameter	SD	N
0.3 ct	4.31 mm	0.037	5736
0.4 ct	4.72 mm	0.038	4536
0.5 ct	5.1 mm	0.042	8225
0.6 ct	5.4 mm	0.039	2922
0.7 ct	5.68 mm	0.042	4363
0.8 ct	5.93 mm	0.041	2768
0.9 ct	6.17 mm	0.045	1967
1.0 ct	6.4 mm	0.051	2271
1.1 ct	6.61 mm	0.046	523
1.2 ct	6.8 mm	0.051	882
1.3 ct	6.98 mm	0.047	795
1.4 ct	7.15 mm	0.049	579
1.5 ct	7.32 mm	0.052	872
1.6 ct	7.48 mm	0.052	182
1.7 ct	7.62 mm	0.047	534
1.8 ct	7.77 mm	0.057	351
1.9 ct	7.91 mm	0.056	188
2.0 ct	8.05 mm	0.059	270
2.1 ct	8.2 mm	0.061	67
2.2 ct	8.33 mm	0.064	71
2.3 ct	8.43 mm	0.056	46
2.4 ct	8.56 mm	0.066	70
2.5 ct	8.67 mm	0.067	188
2.6 ct	8.79 mm	0.065	49
2.7 ct	8.9 mm	0.07	79
2.8 ct	9.01 mm	0.071	23
2.9 ct	9.09 mm	0.064	22
3.0 ct	9.2 mm	0.057	46
3.1 ct	9.32 mm	0.041	16
3.2 ct	9.41 mm	0.059	18
3.3 ct	9.52 mm	0.04	17
3.4 ct	9.63 mm	0.056	20
3.5 ct	9.68 mm	0.07	30

Round diamond carat to mm conversion figure