Bike Tyre Circumference & Wheel Size Calculator

How it works

Outside diameter ≈ bead-seat diameter + 2 × (0.95 × tyre width + 1 mm); circumference = π × diameter; rolled-out ≈ circumference × (1 − deflection), deflection ≈ 1–3% by load and pressure; revolutions per km = 1,000,000 ÷ circumference

Bike computers and some GPS units measure distance by counting wheel revolutions, so they need the circumference of the tyre as it rolls under the rider's weight. The ETRTO size printed on the sidewall gives the rim's bead-seat diameter and the tyre's nominal width, from which the outside diameter follows because an inflated tyre is roughly as tall as it is wide; the load then squashes it by one to three percent depending on weight and pressure, which is why a measured roll-out beats any table. Traditional inch sizes are notoriously ambiguous — four different "26-inch" rims exist — so the two-number ISO designation is the one to trust. A one percent error in circumference is a one percent error in every speed and distance the computer ever shows.

Worked example

A 28 mm tyre on a 622 mm (700c) rim has an outside diameter of about 677 mm and a geometric circumference of 2,127 mm; at 85 kg and 80 psi it rolls out about 1.2% shorter, around 2,102 mm. A computer programmed with the table value of 2,136 mm would read 1.6% high — 101.6 km for a 100 km ride.

Frequently asked questions