Ring Size from Diameter Calculator

About this calculator

A ring's size ultimately comes down to the inner circumference of the band, which is derived directly from its inner diameter using the basic circle formula circumference = π × diameter. Because that circumference maps onto a standardized sizing scale, jewelers can convert a single diameter measurement — typically taken by slipping a ring onto a graduated mandrel or measuring an existing ring that fits — into a US ring size using published conversion tables.

Jewelers, ring resizing services, and online jewelry buyers use this diameter-to-size conversion constantly, especially for cross-checking a customer's self-measured ring against a printed ring sizer, converting between international sizing systems (US, UK, and EU sizes all use different scales for the same physical circumference), or verifying a size before an online purchase where trying the ring on isn't possible.

This calculator takes a ring's inner diameter in millimeters and returns the corresponding circumference and US ring size.