How to use the Tire Size Calculator
The tire size calculator finds a tyre's diameter, sidewall height and revolutions per mile.
Open the Tire Size Calculator →
What you need to enter
- Width (mm)
- Aspect ratio (%)
- Rim diameter (in)
The formula
Diameter = rim + 2 × (width × aspect ratio ÷ 100) ÷ 25.4.
Worked example
Using the default values in the calculator:
- Width (mm): 225
- Aspect ratio (%): 45
- Rim diameter (in): 17
| Overall diameter | 24.97 in / 634 mm |
| Sidewall height | 101.3 mm |
| Circumference | 78.45 in |
| Revolutions per mile | 807.6 |
Tips for better results
- Keep new tyres close to the original diameter.
- Check speedometer effects.
Common mistakes to avoid
- Mixing mm and inches.
- Ignoring load and speed ratings.
Please note: Results are estimates for general use. For safety-critical work such as electrical or building projects, check with a qualified professional and local codes. See our Disclaimer.