Tire Size Calculator
Calculate tire diameter and circumference from its size code.
About the Tire Size Calculator
Decodes a tire size like 225/45R17 into the overall diameter, sidewall height and circumference, in millimetres and inches.
How to use
Enter the width, aspect ratio and rim diameter from the tire code, then click Calculate.
Worked example
225/45R17 → overall diameter ≈ 634 mm (25.0 in).
How to use this calculator
- Enter the tire size code into the calculator.
- Read the tire diameter and circumference from the results.
- Use the worked example to check how the size code is interpreted.
The formula explained
$$ \text{diameter} = \text{rim} + 2 \times \text{sidewall}, \quad \text{sidewall} = \frac{\text{width} \times \text{aspect ratio}}{100}, \quad \text{circumference} = \pi \times \text{diameter} $$
- \(\text{diameter}\) = overall tire diameter
- \(\text{rim}\) = wheel diameter in inches
- \(\text{sidewall}\) = one sidewall height in the same unit as the rim after conversion
- \(\text{width}\) = tire section width in millimeters
- \(\text{aspect ratio}\) = sidewall height as a percentage of the tire width
- \(\text{circumference}\) = distance around the tire
- \(\text{pi}\) = the constant pi, about 3.14159
Step by step method
- Find the width, aspect ratio, and rim diameter in the tire code.
- Convert the sidewall height from millimeters to inches if needed, then add the two sidewalls to the rim diameter.
- Multiply the final diameter by pi to get the circumference.
Worked example
Here is a worked example for a common tire size, 205/55R16.
- The width is 205 mm and the aspect ratio is 55, so the sidewall height is \(205 \times 0.55 = 112.75\) mm.
- Convert the sidewall to inches: \(112.75 \div 25.4 \approx 4.44\) in.
- Compute the diameter: \(16 + 2 \times 4.44 \approx 24.88\) in.
- Compute the circumference: \(3.14159 \times 24.88 \approx 78.19\) in.
Answer. Diameter is about 24.88 inches and circumference is about 78.19 inches.
Tips and common mistakes
- Check that you are reading the tire code correctly, especially the rim size at the end.
- Remember that the width is in millimeters, while the rim diameter is in inches.
- If you compare two tire sizes, even a small change in sidewall height can affect the total diameter.
- Use the calculator for an exact result instead of estimating by eye.
Frequently asked questions
What do the numbers mean?+
Width(mm)/Aspect(%)R Rim(inches): e.g. 225/45R17.
Why does tire size matter?+
It affects speedometer accuracy, clearance and ride; keep replacements within ~3% diameter.
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