Tyre Size & Circumference Calculator
Enter a tyre’s ETRTO width and rim diameter (the two numbers moulded on every sidewall, like 47-622) and get its true circumference — the number your bike computer needs to measure speed and distance properly.
Your numbers
Overall diameter ≈ rim diameter + twice the tyre width — an approximation that treats the tyre’s height as equal to its width.
Result
How it works
Every bike computer measures speed by counting wheel revolutions and multiplying by a circumference you give it. Get that number wrong by 3% and every ride is 3% longer or shorter than it really was — which, over a year of riding, quietly invents or deletes entire weeks of distance. This calculator derives circumference from the ETRTO size: rim bead diameter plus twice the tyre width gives overall diameter, and π does the rest.
Real tyres sit slightly proud or slightly squat depending on rim width and pressure, so the rollout test beats any formula: mark the valve at the floor, roll the bike one full wheel turn in a straight line under your weight, and measure. If that number differs from this page, use yours.
Good practice & limits
- Re-enter the circumference after any tyre change — a 25c to 32c swap changes it by about 50 mm.
- Under-inflated tyres roll a slightly smaller circle. Another reason pressure matters.
- GPS computers do not need this number; sensor-based ones are only as honest as it is.