1 - Cut a string or floss around 100 mm.
2 - Wrap around the base of your finger. 
3 - Mark the point on the string where it overlaps forming a circle.
4 - Compare the length of the string to the chart attached.

*If you are between sizes, order a size up. It is always better to resize the ring in the future rather than purchasing a ring that is too small.Make sure your finger is at a normal body temperature (fingers shrink or expand when cold or hot).