What ring size is 21 mm?
Approximately US size 11.521 mm inside diameter equals 66.0 mm inside circumference.
How this conversion is calculated
The circumference is diameter × π. The US result is rounded to the nearest quarter size because jewellers normally sell discrete sizes rather than an infinitely precise diameter.
Measure only the empty inside of the ring at its widest point. Even a small metal-edge measurement error can change the recommended size.
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