Boneyard Tools

How to measure your wheel circumference (rollout)

Wheel circumference is the single input that ties cadence and gearing to real speed. Here is how to measure your rollout accurately in metres.

Open the Cycling Speed Calculator →

Why circumference matters as much as gearing

The gear ratio decides how many times the wheel turns per pedal stroke, but the wheel circumference decides how far each of those turns actually carries you. Two bikes in an identical 50x15 gear at the same cadence will roll at different speeds if their tires differ in size. That is why the calculator asks for circumference in metres rather than assuming a wheel diameter, and why a careful measurement makes the speed figure trustworthy.

The roll-out method, step by step

The most accurate way to find circumference is the roll-out test. Inflate the tire to your normal riding pressure and sit on the bike so it is under real load. Mark the valve position on the floor, roll forward exactly one full wheel rotation until the valve returns to the bottom, and mark the new spot. Measure the distance between the two marks in metres and that is your rollout. Doing it under load matters because a squashed tire has a slightly smaller effective circumference than a free-spinning one.

Typical values you can start from

If you cannot measure right now, use a published estimate and refine it later. A 700x23c road tire is close to 2.096 m, a 700x25c is about 2.105 m, and a 700x28c sits near 2.136 m. A 650b gravel wheel is roughly 2.00 m, and a 26 inch mountain tire is around 2.07 m depending on tread and width. Enter the closest value, then swap in your measured rollout once you have it for a sharper result.

Keeping the number honest over time

Circumference drifts as conditions change. A softer tire, a heavier rider, a new set of rubber with deeper tread or a switch to a wider casing all move the effective rollout by a few millimetres. Those millimetres are small per turn but add up across thousands of revolutions on a long ride. Re-measure whenever you change tires or pressure noticeably, and the calculator will keep matching what your bike computer reports.

Frequently asked questions

Can I calculate circumference from the tire size instead?

You can approximate it. Multiply the overall wheel and tire diameter in metres by pi (about 3.1416). This is close, but a measured roll-out under your weight is more accurate because it accounts for the tire squashing under load.

Should I measure in millimetres or metres?

Measure in whatever is convenient, then convert to metres for the tool. A rollout of 2105 mm is 2.105 m. The calculator expects metres, so divide a millimetre measurement by 1000 before entering it.

Does tire pressure really change the result?

Slightly. Lower pressure flattens the contact patch and shortens the effective rollout by a few millimetres, which nudges the calculated speed down. For consistent numbers, measure at the pressure you normally ride.