Tyre Size and Circumference Calculator
Calculate a tyre's rolling circumference and diameter from its size code, such as 225/45R17, in millimetres and inches.
Your results
Calculation breakdown
- Diameter
- Diameter = (rim × 25.4) + 2 × (width × aspect ratio / 100)
- Circumference
- Circumference = diameter × π
Worked example
A 225/45R17 tyre has an overall diameter of about 627mm and a rolling circumference of about 1,970mm.
Assumptions
- Assumes the tyre size code is standard metric format.
- Actual manufactured tyres vary slightly around published specifications.
How this calculator works
Tyre size codes like 225/45R17 encode the tyre's width, sidewall aspect ratio and rim diameter, which together determine the tyre's overall diameter and rolling circumference. Enter these three figures to calculate the tyre's diameter and circumference, useful when checking speedometer accuracy after a tyre or wheel change.
Frequently asked questions
How do I read a tyre size code?
In 225/45R17, 225 is the tyre width in millimetres, 45 is the sidewall height as a percentage of width (aspect ratio), and 17 is the rim diameter in inches.
Why does tyre size affect my speedometer?
Changing to a tyre with a different overall diameter changes how far the car travels per wheel rotation, which can make your speedometer and odometer read slightly inaccurately.
What is rolling circumference used for?
It helps estimate revolutions per kilometre, useful for gearing calculations, off-road tyre upgrades or checking compatibility with a different tyre size.