Tire Size Calculator
Decode a tyre size and work out its actual dimensions and differences between sizes.
Reading the numbers on the sidewall
A tyre size like 205/55R16 packs the width, the profile and the wheel size into one code that means little until you decode it. This breaks a tyre size into its real dimensions and can compare two sizes, so you can understand what is on your car, check whether an alternative size will fit, or see how a change affects the tyre's overall diameter.
Enter a tyre size and see what it actually means.
Why the overall diameter matters
The reason comparing sizes matters is that the tyre's overall diameter affects real things: fit within the wheel arch, and the accuracy of your speedometer, which is calibrated to a specific rolling diameter. Fit a tyre with a noticeably different diameter and your speedometer reads wrong and the odometer miscounts. That is why, when changing tyre or wheel sizes, people aim to keep the overall diameter close to the original — and why decoding and comparing the numbers, rather than guessing, is worth doing before buying.
Tire Size Calculator FAQ
What do the numbers in a tyre size mean?
They encode the width, the sidewall profile as a percentage, and the wheel diameter — for example 205 wide, 55 profile, 16-inch wheel.
Why keep the overall diameter similar when changing size?
Because it affects fit and speedometer accuracy — a very different diameter makes your speed and mileage read wrong.
Is anything uploaded?
No. The calculation happens in your browser.