Tire Rolling Circumference Calculator
Enter the tire width, aspect ratio and rim diameter to get the rolling circumference and revolutions per km.
Result
How it is calculated
Read a size such as 225/45R17 as width W in millimetres, aspect ratio A in percent and rim diameter R in inches. The sidewall height is s = W × A ÷ 100 mm. The overall diameter is D = R × 25.4 + 2s mm, because there is a sidewall above and below the rim. The static circumference is π × D. The tool multiplies it by 0.97, an assumed 3 % loss from the tyre flattening under load, to get the rolling circumference. Revolutions per km are 1,000,000 ÷ rolling circumference. Inputs must look like real markings: width 50–1000 mm, aspect ratio 10–100 %, rim 4–60 in. Anything outside those ranges shows no result. The 3 % factor is a fixed approximation, not a measurement of your tyre.
Worked example
Tyre 225/45R17. Sidewall: 225 × 45 ÷ 100 = 101.25 mm. Rim: 17 × 25.4 = 431.8 mm. Overall diameter: 431.8 + 2 × 101.25 = 634.3 mm. Static circumference: π × 634.3 ≈ 1,992.7 mm. Rolling circumference: 1,992.7 × 0.97 ≈ 1,932.9 mm. Revolutions per km: 1,000,000 ÷ 1,932.9 ≈ 517. For comparison, a 205/55R16 has a diameter of 631.9 mm, about 0.4 % smaller, so a speedometer fitted for the first size would read about 0.4 % high on the second.
Common mistakes
- Entering the rim in millimetres or the width in inches. The width is in millimetres and the rim in inches, as printed on the tyre wall; a rim of 431.8 is refused because it is outside 4–60.
- Treating the aspect ratio as a height in millimetres. It is a percentage of the width, so 45 means a sidewall 45 % as tall as the tyre is wide; values below 10 or above 100 are refused.
- Using the static circumference for speedometer or odometer comparisons. The rolling figure matches how far the car travels per turn, and it is about 3 % smaller here.
- Entering a width such as 22.5 for a 225 tyre. Widths under 50 mm are rejected as an impossible tyre.
- Comparing only the width. Two tyres with the same width but different aspect ratios or rims can have quite different circumferences.
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 — that diameter multiplied by pi gives the static circumference, the rolling circumference is taken as 97% of it (allowing for about 3% tire deflection under load), and revolutions per kilometre is 1,000,000 divided by the rolling 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 and static circumference, overall diameter and revolutions per kilometre.
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