1st method: using a ring you already own (diameter)
You already have a ring on hand? Then simply measure the inside diameter of the ring. You can then find your size by converting using the table above.
If you can't find your exact size, then take the next size up.
2nd method: measure the circumference of your finger
To find out your ring size, simply take a piece of wire, dental floss or paper. Wrap it loosely around the spot on your finger where you want to wear the ring and mark where it overlaps. Then measure the length. The millimeters measured reflect the inner circumference of the ring.
If you can't find your exact size in the chart above, then take the size that is above. For example, if your finger circumference is 57mm, then take the size 58.