Ring Size Calculator – diameter mm to US/UK size

Measure the inner diameter of a ring that fits — in millimetres — and the calculator derives both major systems. US sizes run in numbers and halves, UK sizes in letters and halves, and the circumference π × d is shown for string-measuring. Jewellers' tolerances are about a quarter size, so round to the nearest half size. For a comfort fit or wide band, go half a size up.

1. 1

   Step 1

   Measure the ring's inner diameter in mm

2. 2

   Step 2

   Enter the diameter

3. 3

   Step 3

   Read the US and UK sizes

4. 4

   Step 4

   Check the circumference against a string measurement