What do you need
To find your size using this method, all you need is a ruler, scissors, and some thread or string.
Cut a piece of thread and wrap it around the finger whose size you want to know. Take care to choose the exact spot where the ring will go and do not overtighten.
Cut the string where it wraps around your finger, take the circumference measurement on the ruler, and compare it to the table at the bottom of the page.
For this other method you need two things: a printer and a ring that fits comfortably.
Download and print the table you find at this link, and follow the instructions written on it.
Or you can find your size by using a precision caliper and measuring the diameter of your finger.
Always in the same table you will find the size compared to the diameter.