The easiest way is to measure an existing ring that you have at home and that fits well. Measure the diameter of the ring in the inner dimensions where it is at its widest. Use a a slider, tape measure or a ruler. Round up to the ring size that best suits our table.
Example: A ring that I wear often and am comfortable with measures 18.3 mm. In order to be as good as my new Edblad ring, I will therefore buy a size L.
You can also measure the circumference of your finger to find out which ring size fits best. Then take a piece of string and wrap around your finger. Cut off the string when it is wound one turn. Then measure how far the string is. Round up to the ring size that best suits our table.
Example: After wrapping a string around the fingers I cut off the string where it has been twisted a full turn, I now have a piece of string. The string measures 51.2 mm and in order to be comfortable with my new Edblad ring I will therefore buy a size S.