Tire Size Calculator
Estimate nominal tyre diameter, circumference, and revolutions per mile.
How the result changes
Only section width changes; all other inputs stay at your current values. The line illustrates the formula across the shown range. This is not a forecast of real-world outcomes.
About this calculation
A metric tyre code joins millimetre section width, percent aspect ratio, and inch wheel diameter. This calculator decodes those three numbers into nominal sidewall height and rolling geometry for comparing sizes. Estimate tire size when planning a trip or vehicle expense.
Formula
diameter in inches = rim inches + 2 × (section width mm × aspect/100) ÷ 25.4
Worked example
With Section width: 225 mm, Aspect ratio: 45 %, Rim diameter: 17 in, sidewall height is 101.25 mm.
Inputs and units
For a marking such as 225/45R17, enter 225 millimetres, 45 percent, and 17 inches. The R indicates radial construction but is not a numeric input in this geometry calculation.
- Section width (mm): Read the first number on a metric tyre marking, such as 225/45R17.
- Aspect ratio (%): Read the number after the slash as sidewall height relative to section width.
- Rim diameter (in): Read the last number in the marking as bead-seat wheel diameter.
Calculation and worked example
Sidewall height is section width multiplied by aspect ratio divided by 100. Convert that millimetre height to inches, double it for both sides of the wheel, and add rim diameter. Circumference is π times diameter, and one mile contains 63,360 inches In compact notation, diameter in inches = rim inches + 2 × (section width mm × aspect/100) ÷ 25.4.
A 225/45R17 tyre has a nominal 101.25 mm sidewall. Its diameter is 17 + 2 × 101.25/25.4, about 24.972 inches. Circumference is about 78.45 inches, giving roughly 808 nominal revolutions per mile.
How to use the answer
These are nominal unloaded dimensions derived from the printed code. Real rolling circumference under vehicle load can differ, so the revolutions-per-mile result is a comparison estimate rather than a calibration for an odometer or safety system.
Compare two possible tyre sizes by running each marking. Diameter differences can affect clearance and indicated speed, but fitment also depends on rim width, load rating, brake clearance, and vehicle guidance.
Assumptions and checks
The shape is treated as a circle with exactly the marked dimensions, and no tread wear, inflation, or load deformation is modelled. Section width is not the same as tread width.
Do not add the aspect ratio as 45 millimetres; it is 45% of the 225 mm section width in the example. The rim size is inches, so convert the sidewall before adding it.
A 100% aspect ratio on 25.4 mm section width adds one inch of sidewall on each side. That simple case checks the conversion and the factor of two.
Common questions
What does the sidewall height result represent?
It is the geometric unloaded tyre diameter predicted from the printed size code, not a measurement of the tyre mounted and carrying a vehicle.
What should I check before relying on this result?
Verify all three marking numbers and consult vehicle and tyre guidance before changing sizes; nominal revolutions per mile are not a fitment approval.
Inputs
- Section width (mm)
- Aspect ratio (%)
- Rim diameter (in)