A sensor-based bike computer knows exactly two things: how many times your wheel turned, and a circumference figure you gave it — possibly years ago, possibly from a chart for a different tyre. Multiply a wrong circumference by thousands of revolutions and every ride you record is systematically too long or too short. This article shows you how to read the size code moulded on your own tyre, turn it into a circumference you can trust, and verify it with the sixty-second rollout test.
Historical footnote: before GPS, circumference was the whole of cycling measurement, and riders guarded their rollout numbers like racing lines. The skill has faded because satellites are easy — but satellites sample, tunnels happen, and the wheel sensor remains the only instrument that was actually there for every metre.
Reading the sidewall properly
Every modern tyre carries an ETRTO designation: two numbers in millimetres, width first, bead-seat diameter second — 25-622, 47-584, 60-559. This is the measurement to trust. The inch markings beside it (‘700×25c’, ‘29×2.3’) are traditional names, and the inch ‘diameter’ they imply is historical fiction for most sizes. Our tyre calculator wants the ETRTO pair because the arithmetic is honest from it: overall diameter is bead diameter plus twice the width, and circumference is that times π. A 47-622 gravel tyre works out near 2,246 mm — a number no generic chart for ‘700c’ would have given you.
Key takeaways
- Trust the ETRTO pair in millimetres, not the inch name
- Circumference = π × (bead diameter + 2 × width)
- A wrong setting falsifies speed and distance by the same percentage
How big is the error, really?
Swap a 25 mm road tyre for a 32 without updating the computer and the circumference grows about 50 mm — 2.4%. Your recorded 40 km ride was 41. Over a 5,000 km year the missing distance is 120 km, and every ‘average speed’ comparison across the swap is quietly corrupted. Charts compound the problem by printing one circumference per nominal size while real tyres of the same label vary by brand, rim width and pressure. None of this matters for a Sunday spin; it matters for training logs, for odometer-based maintenance, and for anyone who has ever argued about whether a ride was 98 or 102 kilometres.
| Tyre (ETRTO) | Circumference ≈ | |
|---|---|---|
| 25-622 | 2 | 105 mm |
| 32-622 | 2 | 155 mm |
| 47-622 | 2 | 246 mm |
| 47-584 | 2 | 076 mm |
| 60-622 | 2 | 329 mm |
The rollout test: the final authority
Chalk a line on the tyre at the valve. With you seated on the bike — your weight compresses the tyre and shrinks its effective circle — roll straight for exactly one revolution until the valve returns to the bottom, and measure the distance on the ground. Do it three times, average, and enter that. This measures your tyre, on your rim, at your pressure, under your mass: the four variables formulas approximate. It takes a minute and ends all arguments, including with this article.
Rim width: the variable hiding in plain sight
The same labelled tyre measures differently on different rims. A wide rim stretches the tyre’s casing outward, making it slightly taller and wider than on a narrow rim — the ‘lightbulb’ versus ‘teardrop’ profiles of tyre-rim pairing debates. The circumference effect is small, a few millimetres, but it is systematic, and it means a circumference measured on last year’s wheels is not quite this year’s number after a wheel upgrade. It is also why two riders with ‘identical’ 28 mm tyres can record different distances on the same ride and both be right about their own wheels.
The rollout test absorbs all of this silently, which is its enduring virtue: it does not care why the circle is the size it is.
What to do when GPS and wheel disagree
On open roads, a well-set wheel sensor and GPS typically agree within a percent; in cities, tunnels and tree cover, GPS traces cut corners and invent distance in the other direction. When the two disagree persistently in clear conditions, one of them is miscalibrated and the rollout test is the referee — measure the wheel properly, and whichever device contradicts it loses. For riders logging training seriously, consistency matters more than the last decimal: pick the corrected wheel figure, keep it across the season, and your trend data becomes internally honest even if the universe disagrees about the third significant figure.
And keep the old number written down somewhere when you change tyres. Future-you, setting up a new computer in a hurry, will be grateful — the sidewall gives you the formula’s inputs, but the rollout you measured was the truth.
Odd sizes and historical traps
A few sizes exist to ambush the unwary. The ‘28 inch’ label covers at least three different bead diameters (635, 642 and the 622 of a 700c wearing a fat tyre), so a chart keyed to inches can be wrong by nearly a decimetre of circumference while looking authoritative. French 650 sizes split similarly between 584 and 571. The ETRTO pair is the only dialect immune to this archaeology, because it measures the rim the tyre actually seats on — which is why manufacturers mould it first and why this site’s calculator insists on it. If your sidewall shows only an inch designation, the bead diameter can be measured directly: across the rim’s bead seat, or read from the rim’s own markings, which follow the same standard.
Vintage bikes add one more wrinkle — tyres for obsolete bead diameters are reproduced in small runs with period sizing — but the rollout test remains gloriously indifferent to history. The wheel turns; you measure; the argument ends.
Pressure, the silent variable
A soft tyre rolls on a smaller effective radius, which is why the same wheel can measure a few millimetres less in circumference on a low-pressure day. The effect is small — well under a percent across sensible pressures — but it is one more reason the rollout test at your riding pressure beats any table, and why obsessive accuracy should be aimed at consistency rather than at decimals. Set it once, properly, then ride.
Common mistakes
- Using the inch size name to pick a chart value
- Leaving the old setting after a tyre change
- Measuring the rollout unweighted, standing beside the bike
Questions this raises
Auto modes calibrate from GPS over distance and are usually close — verify once against a rollout or a measured kilometre and then stop worrying.
Slightly: a worn tyre is a fraction of a millimetre smaller in radius. Over a tyre’s life the effect is under half a percent — real, and ignorable.
Put a number on it.
A wheel-size setting from the wrong table quietly falsifies every ride. Reading your ETRTO code, computing its circumference, and the rollout test that settles all arguments.
Open the Tyre Size & Circumference Calculator