Tyre Size Calculator
Calculate the dimensions of a tyre from its code (e.g. 205/55 R16).
Tip: 205/55 R16 means: width 205 mm, profile 55%, rim 16 inches. Overall diameter ≈ 63 cm.
Did this tool help?
About this digital tool
The tyre size calculator works out the real dimensions of a tyre from the code written on it, such as 205/55 R16. Enter the width in mm, the aspect ratio as a percentage and the rim diameter in inches, and get the overall diameter, the sidewall height, the circumference and the revolutions per kilometre.
The sidewall height is width × aspect ratio. The overall diameter is the rim diameter (inches × 25.4 mm) plus twice the sidewall, while the circumference comes out as π × diameter. Revolutions per km are calculated as 1,000,000 mm divided by the circumference. It is useful when comparing two tyre sizes, when changing rims or when you want to check that the new tyre fits into the wheel arch.
How to use
- Find the tyre code on the sidewall, e.g. 205/55 R16.
- Enter the width in mm and the aspect ratio in %.
- Enter the rim diameter in inches.
- Read the overall diameter, the sidewall, the circumference and the revolutions per km.
Frequently asked questions
How do I read a tyre code?
E.g. 205/55 R16: width 205 mm, profile height 55% of the width (112.75 mm), fits a 16-inch rim. R = radial.
What does the code 205/55 R16 show?
205 is the tyre width in mm, 55 is the aspect ratio (the sidewall height as a percentage of the width) and R16 shows radial construction for a 16-inch rim. The tool uses these three values for the calculation.
How do I compare two tyre sizes?
Calculate the overall diameter of both and compare. If the difference is small, the speedometer and odometer are barely affected; large differences change the readings and may rub against the wheel arch.
Why do revolutions per kilometre matter?
They show how many times the tyre turns in 1 km. A tyre with a smaller diameter turns more, so the speedometer may read higher than the real speed. Compare this value between two tyres.