The Following table gives an approximate guide to the relative diameter size in millimeters, and weight of modern round brilliant cut diamonds of ideal proportions. Since 1914, the metric diamond carat is defined as one-fifth of one gram.

 

Ring Size Conversion table
Ring Size MM  Inches
7 14.86   .585
8 15.27   .601
9 15.70   .618
10 16.10   .634
11 16.51   .650
12 16.92   .666
13 17.35   .683
14 17.75   .699
15 18.19   .716
16 18.59   .732
17 18.99   .748
18 19.41   .764
19 19.84   .781