bikecalcs.uk
Tyre Rollout Calculator
Calculate wheel circumference for accurate bike computer setup.
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
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.
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.
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.
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.
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Last updated: June 2026
All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.