Bike Tire Circumference Calculator

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: 2290
• 29 × 2.40: 2330

Sanity-check any of these against the rim standard and it comes out close. ISO gives the rim a bed diameter in millimetres, the tyre adds its width top and bottom, and π turns that into a rollout. A 700c rim is 622 mm, so a 25 mm tyre is 622 + 50 = 672 mm across and 2,111 mm around. A 29er rim is the same 622, and a 2.10 inch tyre is 53 mm, so 622 + 106 = 728 mm and 2,287 mm around.

The measured tables published by Sigma, Garmin and Wahoo sit below the ISO arithmetic on wide knobbly tyres, by up to about 30 mm, because a loaded tyre squashes at the contact patch and the rolling circumference shrinks with it. Road figures barely move. Your own rollout will usually land within 10 to 15 mm of the list above, and where it does not, your rollout is right and the list is wrong.

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? On a 2,110 mm wheel a 1 mm error is 0.047% of the circumference, and every reading on the computer is off by that same fraction: 0.014 km/h at 30 km/h, and 47 metres over a 100 km ride. Most casual riders will never notice. If you are chasing sprint segments on Strava, it is the difference between a personal record and the same time as last week.