Last updated October 13, 2026 · Report an error
A tire's metric size code (like 225/45R17) encodes its width, aspect ratio, and wheel diameter — but not its overall size directly.
This calculator converts that code into the tire's actual sidewall height, overall diameter, circumference, and revolutions per mile.
How it works
Enter the tire's width (mm), aspect ratio (%), and wheel diameter (in) from its size code. The calculator finds the sidewall height, adds it twice to the wheel diameter to get the overall diameter, then derives circumference and revolutions per mile.
- Enter tire width (mm).
- Enter aspect ratio (%).
- Enter wheel diameter (in).
- Click Calculate to see your results.
Examples
225/45R17
A 225/45R17 tire has a sidewall height of 101.25 mm and an overall diameter of about 24.97 inches, resulting in roughly 808 revolutions per mile.
Who should use it
- Checking whether a different tire size will affect speedometer accuracy before buying.
- Comparing the overall diameter of two candidate tire sizes for a vehicle.
Industry applications
- Automotive maintenance and tire shops
- Vehicle customization and off-roading
Advantages
- Converts a tire size code directly into real-world diameter and circumference measurements.
- Includes revolutions per mile for speedometer accuracy checks.
Limitations
- Assumes the sizing follows the standard metric passenger tire code format.
Common mistakes to avoid
- Mixing up width and wheel diameter — width is in millimeters, wheel diameter is in inches.
- Forgetting that aspect ratio is a percentage of width, not a fixed measurement.
Best practices
- When comparing tire sizes, check that the overall diameter difference stays within about 3% to avoid speedometer and clearance issues.
Tips
- Keep replacement tires within about 3% of the original overall diameter to avoid speedometer inaccuracy and drivetrain strain.