Round Diamond Carat Size Chart

We’ve estimated the average diameter (mm) for excellent cut round diamonds using measurements listed on Blue Nile. 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