See the below image for the Ring size conversion diagram. Select a ring size in one category, the rest will change to match. Wrap a piece of string or a strip of paper around your finger. Mark the point where the two ends meet. Measure the string or paper against a ruler to get the circumference of your finger. Divide that by 3.14 to get the diameter of your finger.
It shows ring sizes in both mm and inches, plus internal diameter and external diameter. It also shows conversions between the numerical (USA, Asia) and alphabetical (Australia, UK) ring size measuring systems. The same chart covers ring sizes for both women and men as they both use the same systems of measurement.
Ring sizes in the U.S. range from 3 to 13.5 in adult sizes. Most women’s fingers fall between 3 and 9, while men are typically between 8 to 14. These sizes are actually determined by millimeters.