Tire Size Converter – Compare Tire Sizes Online
Work out overall tire diameter, circumference and revolutions per mile from a tire size.
About the Tire Size Calculator
What is the Tire Size Calculator?
A tire size calculator finds the overall diameter, circumference and revolutions per mile of a tire from its width, aspect ratio and rim diameter.
Why use this calculator?
Changing tire sizes affects your speedometer, gearing and clearance, so knowing the exact dimensions before buying matters.
How to use it
- Enter the tire width in millimetres (the first number).
- Enter the aspect ratio as a percentage (the middle number).
- Enter the rim diameter in inches (the number after R).
Formula
Sidewall height = width × aspect ÷ 100; overall diameter = rim + 2 × sidewall.
A tire marked 225/45R17 is 225 mm wide, has a 45% aspect ratio, and fits a 17-inch rim.
Frequently Asked Questions
What do the numbers on a tire mean?
In 225/45R17, 225 is the width in millimetres, 45 is the sidewall height as a percentage of the width, and 17 is the rim diameter in inches.
Will a different tire size change my speedometer?
Yes. A different overall diameter changes how many times the tire turns per mile, so the speedometer reads high or low unless recalibrated.
How precise are the conversions?
Conversions use standard, internationally defined factors and are rounded for display. They are accurate to the number of digits shown.
Can I convert back and forth without losing accuracy?
Yes. Enter the original value to get an exact inverse conversion; avoid chaining rounded results if you need maximum precision.
Which measurement system is used?
Both metric (SI) and US customary / imperial units are supported and clearly labelled.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.
How accurate are the results?
We use standard, widely accepted formulas and calculate in your browser using double-precision arithmetic. Results are rounded for readability, so treat them as reliable estimates rather than a guarantee. Real-world factors can vary the true figure.
Can I add this calculator to my own website?
Yes. Open this calculator page and use the "Add to your website" box in the sidebar to copy a ready-made iframe snippet, then paste it into any HTML page.
Does it work on a phone or tablet?
Yes. The layout is fully responsive, so it works on desktops, laptops, tablets and mobile phones, and every input adapts to touch screens.
Is the information I type kept private?
Yes. All calculations run locally in your browser. Nothing you enter is uploaded, stored or shared with anyone.