Free calculator
Tire Size Calculator
Convert width, aspect ratio and wheel diameter into tire dimensions.
Formula diameter = wheel diameter + 2 × (width × aspect ratio ÷ 100 ÷ 25.4)
Convert a tire size code into diameter and circumference
In a tire code such as 225/50R17, 225 is width in millimeters, 50 is sidewall height as a percentage of width, and 17 is wheel diameter in inches. Sidewall height is width × aspect ratio ÷ 100. Divide millimeters by 25.4 to convert to inches, then add two sidewalls to wheel diameter.
Worked example
For 225/50R17: sidewall height is 225 × 50 ÷ 100 = 112.5 mm. Diameter is 17 + 2 × 112.5 ÷ 25.4 = 25.86 inches. Circumference is diameter × π, about 81.24 inches.
How to use the result
These are nominal geometric dimensions. Actual installed dimensions can differ. This tool does not calculate load rating, wheel offset, rubbing clearance or approval for a replacement tire.
Sources and assumptions
Calculation methodology. Worked examples use the expressly stated inputs; sample prices are illustrative rather than local quotes.
Use a vehicle service sheet next
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Frequently asked questions
What does the middle number in a tire size mean?
It is the aspect ratio: sidewall height as a percentage of tire width. In 225/50R17, the sidewall height is 50% of 225 mm.
Is a 17-inch tire 17 inches tall?
No. Seventeen inches is the wheel diameter. The tire's overall diameter adds the sidewall above and below the wheel.
Can this calculator confirm tire fitment?
No. It calculates nominal diameter and circumference from the tire code. Check vehicle and wheel requirements separately.
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