Ring Size Calculator

Measure the inside circumference of a ring that already fits, or its inside diameter, and this returns the matching size in the American, British, EU/ISO and Japanese systems along with the sizes either side. The American number is computed from the standard scale of 0.458 inch of inside diameter at size zero plus 0.032 inch per size, and the ISO size is simply the inside circumference in millimetres, while the British letter and the Japanese number are read off a published conversion chart. A third action converts a size you already know into the other systems. Published charts disagree with each other by about half a step and a finger changes size through the day, so this narrows the choice and a jeweller's steel sizer settles it.

Calculator Type
Measured In
Wrap a strip of paper or a piece of string round the base of the finger, mark where it meets and measure the marked length. Sizes on the chart run from 44.2 mm to 69.7 mm.
Widget
Measured In
Lay a ring that already fits on a ruler and measure across the inside, not across the metal. Sizes on the chart run from 14.07 mm to 22.19 mm across.
Widget
Half sizes are fine for the American scale. British letters are not entered here; they are returned in the result.
Widget
Related Tools
Circumference Calculator Bra Size Calculator