Tire Circumference

Free Tire Circumference calculator with clear step-by-step results.

About Tire Circumference

Derives tyre diameter from the sidewall marking - aspect ratio is a percentage of section width, not an absolute measurement - and then reduces the geometric circumference by a loaded deflection factor, because a tyre under load has a slightly shorter effective rolling circumference than its free diameter implies.

Formulas

Overall diameter

Diameter = rim inches x 25.4 + 2 x (section width x aspect ratio)

Rolling circumference

Rolling = pi x diameter x (1 - loaded deflection)

Worked example

With the default values below, Tire Circumference returns Rolling circumference of 2,010.3. Change any input above and every figure here is recalculated in your browser.

Inputs used

Section widthminimum 50 mm: 225 mm
Aspect ratioaccepts 20–95 %: 45 %
Rim diameteraccepts 8–30 in: 18 in
Loaded deflectionaccepts 0–12 %: 3 %

Results

Rolling circumference: 2,010.3 mm
Overall tyre diameter: 659.7 mm
Sidewall height: 101.3 mm
Geometric circumference: 2,072.5 mm

Step by step

1Values used
2Overall diameter
3Rolling circumference
4Rolling circumference= 2,010.3 mm
5Overall tyre diameter= 659.7 mm
6Sidewall height= 101.3 mm
7Geometric circumference= 2,072.5 mm

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is loaded circumference smaller?

The contact patch flattens, so the wheel centre travels slightly less per revolution than the free circumference. Two to four percent is typical for a passenger tyre at correct pressure.

Does this affect my speedometer?

Yes. Changing to a tyre with a different rolling circumference alters indicated speed and odometer readings in direct proportion.