Tyre size calculator.
Read a sidewall marking properly: what the numbers mean, how big the tyre actually is, and what changes if a different size goes on the same rim.
What this assumes
- The figures are geometric, taken from the marked size. A tyre under load deflects, so its true rolling circumference is slightly shorter than the free diameter gives - which is why a manufacturer sometimes publishes a marginally different revolutions per kilometre.
- Load and speed figures come from the published ISO and ETRTO tables. They are the tyre rating, not a recommendation for your vehicle: the load a wheel must carry and the pressure it needs come from the vehicle placard.
- Metric sizes only. A flotation size such as 31x10.50 R15 already states its diameter in inches and is not decoded here.