If you don't know your ring size, this tool can help you.
Roll it around your finger not too tight, not too lose. It has to be able to go over your bone and be comfortable at the same time.
The number which appears in the circle is the circumference of your finger in mm.
Once you've got your circumference, you've got your size.
1 size = 1 mm so it's better to be precise.
This tool can be used several times.