About the cm to mm Ring Size Converter
Ring sizes are based on the inner diameter or inner circumference of the band, almost always specified in millimeters. This calculator converts a measurement you took in centimeters into millimeters, works out the matching circumference or diameter, and estimates a US and EU/ISO ring size from it.
The core formula
The relationship between a ring's inner diameter and inner circumference is fixed by geometry: circumference = π × diameter, where π ≈ 3.14159. A ring with a 16.5 mm inner diameter has a circumference of about 51.8 mm (16.5 × π); a ring with a 54 mm circumference has a diameter of about 17.2 mm (54 ÷ π). Entering either measurement lets the calculator derive the other.
Estimating a ring size
- EU / French / ISO size: equals the inner circumference in millimeters, rounded to the nearest whole number. A 52 mm circumference is EU size 52.
- US / Canada size: increases by roughly 0.83 mm of diameter per whole size (about 0.4 mm per half size). This calculator uses the approximation US size ≈ (diameter in mm − 11.57) ÷ 0.83, rounded to the nearest half size, which tracks published US sizing charts closely across the common adult range (about sizes 3 to 13).
Because physical ring sizers can vary slightly between manufacturers, treat the estimated size as a strong starting point rather than a guaranteed fit — confirm with a plastic or metal ring sizer, or a jeweler, before ordering a ring that can't be resized.
How to measure a ring you already own
Lay the ring flat and measure straight across the inside edge (not including the metal) with a ruler in centimeters — that's the inner diameter. Alternatively, wrap a thin strip of paper around the inside of the band, mark where it overlaps, lay it flat, and measure that length for the inner circumference.