Tire Size Calculator
Free tire size calculator. Enter the tire width, aspect ratio and rim size to get the overall diameter, circumference, revolutions per mile and sidewall height.
About the Tire Size Calculator
Estimate a tire’s nominal diameter, circumference, revolutions per mile and sidewall height from section width in millimeters, aspect ratio and rim diameter in inches. Enter one size at a time; copy results before entering another size to compare them. These geometric figures do not verify wheel or vehicle fitment.
Features
- Nominal diameter from section width, aspect ratio and rim diameter
- Circumference and revolutions per mile
- Sidewall height in inches
- Calculate one tire size at a time
- Copy results for manual comparison
How to use this tool
The Tire Size Calculator works right in your browser — no signup, no installation. Follow these steps:
- Read the three numbers off your tire (e.g. 225/55R17).
- Enter the width in mm, aspect ratio as a percentage, and rim in inches.
- The tool converts the metric width to inches internally.
- Read off diameter, circumference, revs per mile and sidewall.
Frequently asked questions
How do you calculate tire diameter?
Diameter = rim diameter + 2 × sidewall height. The sidewall height is the width (in mm) × aspect ratio, converted to inches by dividing by 25.4. For a 225/55R17 tire that is 17 + 2 × (225 × 0.55 / 25.4) ≈ 26.7 in. The calculator does this for any size.
What do the three tire numbers mean?
For 225/55R17, 225 is the nominal section width in millimeters, 55 is sidewall height as a percentage of section width, R denotes radial construction, and 17 is rim diameter in inches.
How many revolutions per mile is my tire?
Revolutions per mile = 63,360 ÷ tire circumference in inches (since a mile is 63,360 inches). A larger tire covers more ground per turn and has fewer revs per mile, which affects speedometer and odometer readings. The calculator returns this directly.
Will a different tire size affect my speedometer?
Yes. A larger diameter tire travels farther per revolution, so your speedometer will read slower than your true speed. The revs-per-mile result lets you estimate the difference when comparing a new size to stock.
Is this tire calculator free?
Yes, completely free with no signup. All maths runs in your browser — nothing is uploaded or tracked.
Using the results
The Tire Size Calculator estimates quantities from your measurements and the stated assumptions. Check dimensions, product coverage and project requirements before buying materials.