How to Read a Tire Size
Enter a tire size in the standard format (e.g. 205/55R16) to get outer diameter, sidewall height, aspect ratio, circumference, and rotations per km โ instantly calculated.
What Each Number Means
First number (e.g. 205): Section width in mm โ the width of the tire at its widest point.
Second number (e.g. 55): Aspect ratio (%) โ sidewall height as a percentage of section width. Lower numbers mean a flatter, sportier tire.
R: Radial construction โ the standard for nearly all modern passenger tires.
Last number (e.g. 16): Rim diameter in inches โ the wheel size the tire fits.
Formulas Used
Sidewall height = width ร aspect ratio / 100 | Outer diameter = rim ร 25.4 mm + sidewall ร 2 | Circumference = ฯ ร outer diameter | Rotations per km = 1,000,000 mm รท circumference
FAQ
Yes. A larger outer diameter means fewer rotations per km, so your actual speed is higher than what the speedometer reads. A tire within ยฑ3% of stock diameter keeps error within acceptable limits.
Going up one rim inch typically requires reducing the aspect ratio to maintain the original diameter. Use this tool to compare outer diameters of both tires before purchasing.