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 This Free Online Tire Size Calculator
Our free tire size calculator decodes the numbers on a tire sidewall into real dimensions. Enter the three numbers from a P-metric size like 225/55R17 — the tread width in millimeters, the aspect ratio (sidewall as a percentage of width), and the rim diameter in inches — and the tool returns the overall tire diameter, the circumference, the revolutions per mile and the sidewall height. Use it to compare sizes, check speedometer effects, or confirm a tire fits your wheels. Everything runs in your browser — no signup.
Features of Our Tire Size Calculator
- Overall tire diameter from width, aspect and rim
- Circumference and revolutions per mile
- Sidewall height in inches
- Handles any P-metric tire size
- Compare two sizes to plan a fitment
- 100% client-side and private — instant results, no signup
How to Use the Tire Size Calculator
Using this tool is simple and requires no signup or registration. 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 About Tire Size Calculator
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?
In a size like 225/55R17, 225 is the tread width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel 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.
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