Tire Rolling Circumference Calculator

About this calculator

A tire's rolling circumference is calculated from its size markings (for example 225/45R17): the sidewall height in millimeters equals the width multiplied by the aspect ratio percentage, doubled (for both sidewalls) and added to the rim diameter (converted from inches to millimeters), giving the overall tire diameter — the circumference is then that diameter multiplied by pi, and revolutions per kilometre is simply 1,000,000 divided by the circumference in millimeters.

Drivers and mechanics need this number when comparing tire sizes for a plus-size or downsize wheel change, because a tire with a different rolling circumference than the factory-specified size throws off the car's speedometer reading, odometer accuracy, and — on cars with electronic stability control or ABS that rely on wheel-speed sensors — potentially the calibration of driver-assist systems; the standard rule of thumb is to keep the new tire's overall diameter within about 3% of the original to avoid these issues.

This calculator takes your tire's width, aspect ratio and rim diameter and returns the rolling circumference, overall diameter and revolutions per kilometre.