Measure your Ring Size at home with the Paper test
Another useful way to measure your size is to cut a long strip of paper and wrap it around the finger you wish the ring to fit on. Wrap it around until it overlaps and mark with a pen where it meets. It should be not loose, but nice and firm around finger. Measure below the knuckle where a ring would sit. Once marked, lay it out flat on the ruler with the beginning of the paper starting at 0. The marked line gives you the length to make your ring. The ring sizer chart here details how to correspond the length to your size.
For example, the total length measured here is approximately 53mm, which equates to a finger size of J1/2.