Question: how to find your ring size in mm?
Answer: Option one: Use string or floss to measure your finger Then, line that up with a ruler and take down its length in millimeters. With standard ring sizes, every half-size matches up to a 0.4 millimeter increment, starting with size 3 equaling 14 mm, size 3.5 equal to 14.4 mm, size 4 equal to 14.8 mm, and so on.