Ring Size Converter — US / UK / EU from mm
Wrap a string, measure in mm (or use an existing ring's inner diameter), and get all three sizing systems with the nearest standard size highlighted.
Quick answer: Measure your finger with a string or an existing ring, enter millimetres, and get your ring size in US, UK and EU systems instantly. Free, private, works on any device — the string method needs no special tools.
💡 Measure in the evening; size up for wide bands.
How to use
- Wrap a thin string snugly around the finger, mark the overlap, and measure it in millimetres — or measure the inside diameter of a fitting ring and multiply by 3.1416.
- Enter the number; US, UK and EU sizes appear instantly.
- Between sizes? Choose the larger for wide bands and hot weather (fingers swell slightly during the day).
About this tool
Ring sizing systems are just different labels on the same physical measurement: EU size equals the inner circumference in millimetres, while US and UK sizes step along fixed charts. Enter either the circumference (string method) or the inner diameter of a ring that fits (× π = circumference) and the tool matches the nearest standard size in every system, telling you when you fall between sizes so you can size up for wide bands.