The mesure is in mm for example 18,20 mm is 1,82cm.You have to mesure the diameter of one your rings, and there you go, you´ll have your size!

Nº- 10 =      15.70

Nº- 12 =      16.30

Nº- 14 =      16.90

Nº- 16 =      17.50

Nº- 18 =      18.20