How to Calculate Tire Size
What is Tire Size?
Tyre notation (e.g. 225/45R17) encodes tread width in mm, sidewall height as % of width, construction type (R = radial), and rim diameter in inches.
Formula
Diameter (mm) = Rim (inches) × 25.4 + 2 × (Section width × Aspect ratio / 100) | Format: 225/50R17
- W
- Section width (mm)
- AR
- Aspect ratio (%)
- D
- Rim diameter (inches)
- OD
- Overall tire diameter (mm)
Step-by-Step Guide
- 1Sidewall height (mm) = Width × Aspect ratio / 100
- 2Overall diameter (mm) = Rim (in) × 25.4 + 2 × Sidewall
- 3Circumference = π × Overall diameter
- 4Changing tyre size affects speedometer and odometer accuracy
Worked Examples
Input
225/45R17
Result
Sidewall = 101.25mm; Diameter = 634.5mm; Circumference ≈ 1,993mm
Frequently Asked Questions
What is Tire Size?
Tyre notation (e. g
How accurate is the Tire Size calculator?
The calculator uses the standard published formula for tire size. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.
What units does the Tire Size calculator use?
This calculator works with inches, percentages, watts. You can enter values in the units shown — the calculator handles all conversions internally.
What formula does the Tire Size calculator use?
The core formula is: Sidewall height (mm) = Width × Aspect ratio / 100. Each step in the calculation is shown so you can verify the result manually.
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