To take the measurements of the rings, simply subtract 40 from the circumference of the finger in which you intend to insert the ring.

Ring Size Table (ITA):

Circumference (mm):  50   -   Ring size:  10
Circumference (mm):  52   -   Ring size:  12
Circumference (mm):  54   -   Ring size:  14
Circumference (mm):  56   -   Ring size:  16
Circumference (mm):  58   -   Ring size:  18
Circumference (mm):  60   -   Ring size:  20
Circumference (mm):  62   -   Ring size:  22
Circumference (mm):  64   -   Ring size:  24
​Circumference (mm):  66   -   Ring size:  26
​Circumference (mm):  68   -   Ring size:  28
​Circumference (mm):  70   -   Ring size:  30