Use a piece of heavy paper or dental floss and wrap it around the base of your finger.

Use a pen to mark the point on the paper or string where the ends meet.

Use a millimeter ruler to measure the paper or string.

Ring Size Guide

Size Inside diameter (mm) Inside diameter (in)
3 14.07 0.55
3.5 14.48 0.57
4 14.88 0.586
4.5 15.29 0.602
5 15.7 0.618
5.5 16.1 0.634
6 16.51 0.65
6.5 16.92 0.666
7 17.32 0.682
7.5 17.73 0.698
8 18.14 0.714
8.5 18.54 0.73
9 18.95 0.746
9.5 19.35 0.762
10 19.76 0.778
10.5 20.17 0.794
11 20.57 0.81
11.5 20.98 0.826
12 21.39 0.842
12.5 21.79 0.858
13 22.2 0.874
13.5 22.61 0.89
14 23.01 0.906

Remember that the ring has to go over your knuckle so ensure the string is

not too tight.

Another note to remember is that the wider the band the tighter the ring so

for wider type bands, go one size up.