Ring Size from Circumference Calculator

About this calculator

Converting finger circumference to ring size relies on the geometric relationship between a circle's circumference and its diameter, since ring sizing standards — including US sizes, which increment roughly every 0.4mm to 0.8mm of inner diameter — are fundamentally based on the inner diameter of the ring band.

Jewelers and online ring retailers rely on this conversion because customers typically measure their finger with a strip of string or paper, which gives circumference, rather than a specialized sizing tool that gives diameter directly, and because US, UK, and EU ring size systems all map to different diameter or circumference ranges, requiring a lookup against standardized sizing charts.

This calculator takes a finger circumference measurement in millimeters and returns the corresponding US ring size along with the inner diameter of the ring.