How to measure tyre width correctly
Tyre width is the single biggest input into the pressure equation after rider weight — but the number printed on the sidewall is rarely what you actually mount. Here's how to find the real number.
Sidewall print vs measured width
A tyre labelled "28" can measure anywhere from 27 to 31 mm depending on the rim it sits on. Wider internal rim widths spread the bead further apart and inflate the tyre to a larger casing diameter. Always measure on your rim, after the tyre has been inflated and ridden for a few hours so the casing settles.
How to measure
- Inflate the tyre to your normal riding pressure.
- Ride for at least 30 minutes (or leave overnight) so the casing relaxes.
- Set a vernier caliper across the widest part of the tyre — usually mid-sidewall, not the tread.
- Record the number in millimetres. That's the value to enter in the calculator.
Rough rule by rim internal width
| Internal rim width | 28 mm-labelled tyre measures | 32 mm-labelled tyre measures |
|---|---|---|
| 19 mm | 27 – 28 mm | 31 – 32 mm |
| 21 mm | 28 – 29 mm | 32 – 33 mm |
| 23 mm | 29 – 30 mm | 33 – 34 mm |
| 25 mm | 30 – 31 mm | 34 – 35 mm |
Approximate. Always measure your own setup for an accurate number.
Why this matters for pressure
The pressure model scales with tyre volume, and volume grows with the square of width. A 4 mm error in width is enough to shift your recommended pressure by 6–10 psi. Measuring once gets you a number you can trust for every future calculation.
Plug your measured width in
Enter your real, measured tyre width in the calculator for the most accurate pressure recommendation.
Open the calculator →