| US | Diameter mm | Circumference mm | EU (ISO) | Japan |
|---|
US ring size maps to inner diameter as 11.63 + 0.8128 x size (mm). Circumference is diameter x pi. The EU (ISO) size is simply the inner circumference in millimetres, and Japanese size is about circumference minus 40.
Example: US 7 = 17.3 mm diameter = 54.4 mm circumference = EU 54 = Japan 14.
Wrap a strip of paper or string around the base of the finger, mark where it overlaps, measure the length in mm and choose "Circumference (mm)" above. Measure at the end of the day when fingers are largest, and go up a half size for a wide band.
About EU size 54 (54.4 mm circumference).
See also: length converter · shoe size converter · PPI calculator