US ring size 12

US size 12 has an inside diameter of approximately 21.4 mm and an inside circumference of 67.2 mm.

How to check whether size 12 fits

Measure the inside diameter of a ring that already fits the intended finger. Measure straight across the widest inside point—do not include the metal. Compare that measurement with 21.4 mm.

This is a jewellery-size conversion, not a guarantee of fit. Finger size changes with temperature and time of day; wide bands often need extra room.