About the Tire Size Calculator
A tyre code such as 225/45R17 packs three numbers: section width in millimetres, aspect ratio as a percentage of that width, and rim diameter in inches. From those you can work out the overall diameter and rolling circumference, which decide your speedometer reading and effective gearing.
Also known as: tyre size calculator, tire diameter calculator, tire dimensions, rolling circumference, tire-aspect-ratio.
How to use this calculator
- Enter section width, aspect ratio, rim diameter.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
diameter = rim × 25.4 + 2 × (width × aspect ÷ 100)
Worked example
Using the values pre-filled in the calculator above:
- Section width = 225 mm
- Aspect ratio = 45 %
- Rim diameter = 17 inches
the calculator returns:
- Overall diameter: 634.3 mm
- In inches: 24.972
- Sidewall height: 101.25 mm
- Rolling circumference: 1,992.71 mm