Why does rollout matter?

Your bike computer converts wheel revolutions to distance using the rollout value. An incorrect value means inaccurate speed and distance readings. Even 10mm difference causes a 0.5% error, which adds up over long rides.

How do I do a manual rollout test?

With your full riding weight on the bike, mark the tyre and ground at the valve. Ride in a straight line for exactly one wheel revolution. Measure the distance between marks. This is your actual rollout distance.

Does tyre pressure affect rollout?

Yes, lower pressure causes more tyre deformation, slightly reducing the effective circumference. The effect is small (typically 1-5mm) but measurable. Higher pressure gives a more round tyre shape and slightly larger rollout.

Do different tyre brands have different rollouts?

Yes. Even at the same nominal size (e.g. 700x25), different brands and models vary by 5-15mm due to differences in actual tyre dimensions, casing construction and tread thickness. The same tyre also measures wider (and so rolls further) on a wide rim — a nominal-25mm tyre often measures 27-28mm on a modern wide rim. The brand presets here are indicative only; for accuracy do a manual rollout test with your weight on the bike and enter the measured value.

Tyre Rollout Calculator — Free Equipment Tool

Tyre Rollout Calculator

Calculate wheel circumference for accurate bike computer setup.

Tyre SizeSelect size or enter custom dimensions

Tyre Brand/ModelPresets are indicative (rim width, pressure and wear shift them ±5–15mm). For accuracy, do a manual rollout test and enter the measured value.

Inflation Pressure

Applied gas pressure

How We Calculate This

ETRTO Standard

Circumference = π × (Bead Seat Diameter + 2 × Tyre Width). This gives the theoretical (geometric) circumference from ISO 5775 / ETRTO tyre dimensions — for a 700C wheel the BSD is 622mm.

Adjustments

The calculator starts from a brand preset or your measured rollout, then adjusts for inflation pressure (lower = smaller rollout) and rider weight (heavier = more deformation = slightly smaller rollout).

Table vs Geometric Ratio

This is the brand/measured rollout divided by the geometric ETRTO estimate of the same circumference. A value near 1.0 means the two methods agree; it is a sanity check on the preset, not a speed-correction factor to type into a bike computer.

The measured rollout is authoritative

Brand presets are indicative only. Real rolling circumference depends on rim internal width, pressure, wear and load (a nominal-25mm tyre can measure 27–28mm on a modern wide rim), so the geometric figure and any preset can differ from reality by around 1%. A manual rollout test with your weight on the bike is the most accurate method (Sheldon Brown).

Frequently Asked Questions

Last updated: 2026-03-01

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.