Measure the paper up to that mark to find the circumference of your ring. Once you have it, divide the number you got by 3.14 to find the diameter and convert it to a size using the chart below.
For example, i measured my finger and got 56mm circumference, which divided by 3.14 gave me a diameter of 17,83mm which is the equivalent to a US size 7.3/4
If you are between two sizes, take into consideration the season (cold weather usually means skinnier fingers), your knuckle-to-finger ratio and preferred fit, as well as how often you plan on taking it on and off, etc. If still in doubt we recommend you choose the smaller one, as this U shape leaves a bit of extra room and loose rings tend to get misshapen easier.
If you are ordering a ring and are still not completely sure of your size, please email us at email@example.com and we will be more than happy to help you find the perfect fit.