To find out your inner diameter in mm, use a ruler and measure the inner diameter of your existing ring and compare below.
If you don’t have an existing ring, use a string or a piece of paper to wrap around the finger you wish to wear the ring, make a marking, use that length to measure over a ruler to find out the inner diameter and compare below.
If you are in between any of these sizes, you can go up one size. For example, if your size is between 51 and 52, choose 52.