Tire Size Diameter Calculator
A tire's P-metric size, such as 225/45R17, encodes everything needed to work out its overall diameter: the section width, the aspect ratio (sidewall percentage) and the rim diameter. This calculator computes the sidewall height, overall diameter and circumference from those three numbers, so you can compare tire sizes before fitting a new set. Enter the width, aspect ratio and rim diameter to see the dimensions.
Tire diameter formula
sidewall (mm) = section width * (aspect ratio / 100)
diameter (mm) = (rim diameter * 25.4) + (2 * sidewall)
diameter (in) = diameter (mm) / 25.4
circumference (in) = pi * diameter (in)
The sidewall height is a percentage of the width. The overall diameter adds two sidewalls to the rim diameter, converting inches to millimetres at 25.4 mm per inch. Circumference is pi times the diameter.
Worked example
For 225/45R17: sidewall = 225 times 0.45 = 101.25 mm. Diameter in mm = (17 times 25.4) + (2 times 101.25) = 431.8 + 202.5 = 634.3 mm. In inches that is 634.3 / 25.4 = 24.97 inches. Circumference = pi times 24.97 = 78.44 inches.
Tire size diameter: frequently asked questions
How do you read a tire size like 225/45R17?
A P-metric tire size has three parts. The first number (225) is the section width in millimetres. The second number (45) is the aspect ratio: the sidewall height as a percentage of the section width. The R means radial construction, and the last number (17) is the wheel rim diameter in inches. From these three numbers you can compute the overall tire diameter.
How is overall tire diameter calculated?
The sidewall height equals the section width in millimetres times the aspect ratio divided by 100. There are two sidewalls (top and bottom), so the overall diameter equals the rim diameter (converted to millimetres at 25.4 mm per inch) plus twice the sidewall height. Dividing the result by 25.4 converts back to inches.
Why does tire diameter matter when changing tire size?
Overall diameter affects your speedometer reading, odometer accuracy, gearing and ground clearance. A tire with a larger diameter travels further per revolution, so the speedometer will read slower than your true speed, and the odometer will under-count distance. Keeping the new tire's diameter within roughly 3 percent of the original avoids significant errors and clearance problems.
What is tire circumference used for?
Circumference is the distance the tire covers in one full revolution, equal to pi times the diameter. It is the basis for speedometer and odometer calibration and for converting between wheel RPM and road speed. A larger circumference means fewer revolutions per mile, which changes effective gearing.
Official sources
- National Highway Traffic Safety Administration: NHTSA tire safety and labelling.
- National Institute of Standards and Technology: Guide for the Use of the International System of Units (inch to mm).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.