Tire Diameter Calculator
Calculate the overall tire diameter in inches from a standard tire size like 225/45R17.
How to use this tool
- Find the three numbers on your tire sidewall (e.g. 225/45R17).
- Enter section width in mm, aspect ratio %, and wheel diameter in inches.
- The calculator returns sidewall height, overall diameter, and circumference.
Decode your tire size and find the exact outer diameter in inches.
Formula
Sidewall height (mm) = Section width (mm) × Aspect ratio (%) ÷ 100
Tire diameter (in) = Wheel diameter (in) + 2 × Sidewall height (mm) ÷ 25.4
Circumference (in) = Tire diameter (in) × π
How it works
This calculator decodes a metric tyre size (e.g., 225/45R17) into real-world dimensions. The sidewall height is a percentage of the section width, both in millimetres; it is then converted to inches and doubled (for both sides) before being added to the rim diameter. Results match published tyre data sheets within 0.1–0.2 inches, with small deviations due to mould vs inflated dimensions.
Worked example
Worked example
- Tyre size: 225/45R17 — width = 225 mm, aspect = 45%, wheel = 17 in.
- Sidewall height = 225 × 45 ÷ 100 = 101.25 mm.
- Sidewall in inches = 101.25 ÷ 25.4 = 3.986 in.
- Tire diameter = 17 + 2 × 3.986 = 17 + 7.972 = 24.972 in.
- Circumference = 24.972 × π ≈ 78.46 in.
Sidewall = 101.25 mm; Tire diameter = 24.97 in; Circumference = 78.46 in
Key terms
- Section width
- The widest point of a tyre from sidewall to sidewall, measured in millimetres and represented by the first number in a tyre code (e.g., 225 in 225/45R17).
- Aspect ratio
- The tyre's sidewall height expressed as a percentage of section width; the second number in a tyre code (e.g., 45 means the sidewall is 45% of 225 mm).
- Wheel diameter
- The diameter of the rim bead seat in inches, matching the 'R17' part of the tyre code.
- Overall diameter
- The full outside diameter of a mounted, inflated tyre, equal to rim diameter plus twice the sidewall height.
- Circumference
- The distance travelled in one full tyre revolution; equals π × overall diameter and directly affects speedometer calibration.
Frequently asked questions
- How do I read a tire size?
- A tire size like 225/45R17 means: 225 mm section width, 45% aspect ratio (sidewall height), R = radial construction, 17 inch wheel diameter.
- Why does tire diameter matter?
- Changing tire diameter affects speedometer accuracy, gear ratios, clearance, and handling. Plus-sizing (larger wheel, lower sidewall) maintains diameter while improving aesthetics and handling.