Bike Tire Circumference Calculator
Calculate bike tire circumference for cyclometer setup from tire size or rollout test.
Includes ISO 622, 700c, 27.5, 29er, and 26-inch wheels.
Every bike computer needs to know how far the wheel travels in one revolution. Get the number wrong by one percent and your distance and speed readouts drift by the same amount across the entire ride. For a 100 km commute, a 1% error is a kilometre.
The simple formula is:
C = π × D
with D being the outside diameter of the inflated tire. If you have a brand-new tire and a manufacturer spec sheet, that is enough. In practice the rolling diameter depends on inflation pressure, rider weight, casing flex, and tread wear, and the only reliable answer comes from a rollout test.
How a rollout test works. Mark a chalk line across the tire at the contact patch. Wheel the bike forward in a straight line until the same mark touches the ground again, and measure the distance you covered. That distance is your true circumference, in millimetres, accurate to within a few mm. It is the gold standard, the one shop mechanics use when a customer’s odometer numbers look off.
For when you do not have time to roll, here are the typical numbers, in millimetres:
- 700×23c — 2096
- 700×25c — 2110
- 700×28c — 2136
- 700×32c — 2155
- 700×35c — 2168
- 26 × 1.95 — 2050
- 26 × 2.10 — 2070
- 27.5 × 2.10 (650b) — 2148
- 27.5 × 2.40 — 2185
- 29 × 2.10 — 2222
- 29 × 2.40 — 2275
These are averages collected by Sigma, Garmin, and Wahoo across reference tires. Real-world rollout numbers usually come within 10 to 15 mm of these.
A small surprise: rolling resistance changes the effective circumference. A heavily loaded touring bike with low pressure rolls a measurably smaller circumference than the same wheel on a workstand. If you are setting up a power meter or a long-distance odometer, do the rollout test on the loaded bike at your normal riding pressure.
How sensitive is this to mistakes? At 30 km/h, a 1 mm circumference error changes the displayed speed by about 0.014 km/h and the displayed distance by 0.05% per kilometre. Most casual riders will never notice, but if you are tracking efforts on Strava with sprint segments, that is the difference between a personal record and the same time as last week.