What does this tool do?
Ring sizing is based on the inside diameter or circumference of the band. This converter translates a measurement into US, UK letter and EU/ISO sizes, with an adjustment for wide or comfort-fit bands.
The Formula
US size = 1.2 × inner diameter (mm) − 13.8; EU/ISO size = inner circumference (mm); UK letter ≈ 2 × US sizeFrequently Asked Questions
Should I measure diameter or circumference?
Either works if the ring already fits. For a finger, wrap a strip of paper around the base, mark the overlap and measure the length in millimetres to get the circumference.
Why do wide bands need a different size?
A wider band contacts more of the finger and feels tighter. This calculator adds half a size for bands of 7 mm or more, while comfort-fit rings are usually taken a quarter size smaller.