Tire Size Calculator
Enter a standard P-metric tire size (for example, 225/45R17) and this calculator returns the overall diameter, section width, sidewall height, circumference, and revolutions per mile. These figures help you compare tire sizes when upgrading wheels, determine speedometer error after a size change, and verify clearance. The formulas follow the Tire and Rim Association (TRA) P-metric standard, which is the basis for every passenger-car tire sold in the United States.
Tire size formula (TRA P-metric standard)
Sidewall height (in) = (Width_mm x Aspect_ratio / 100) / 25.4
Overall diameter (in) = Wheel_diameter + 2 x Sidewall_height
Circumference (in) = Overall_diameter x pi
Rev/mile = 63,360 / Circumference
Where 25.4 is the number of millimetres per inch and 63,360 is the number of inches per mile. This matches the standard used by tire manufacturers and published in TRA Year Book tables.
How to read a tire size code
- 225: Section width in millimetres measured from sidewall to sidewall at the widest point.
- 45: Aspect ratio. Sidewall height is 45% of the section width (225 x 0.45 = 101.25 mm).
- R: Radial construction. Nearly all modern passenger tires are radial.
- 17: Wheel (rim) diameter in inches.
- The load index and speed rating follow the rim size, e.g., 94W. These are not used in the diameter calculation.
Tire size calculator: frequently asked questions
What do the numbers in a tire size mean?
A tire code such as 225/45R17 means: 225 mm section width, 45% aspect ratio (sidewall height as a percentage of width), R indicates radial construction, and 17 inches is the wheel diameter. The overall diameter is calculated from these three values.
How is tire overall diameter calculated?
Overall diameter = wheel diameter (inches) + 2 x sidewall height. Sidewall height = (section width in mm x aspect ratio / 100) converted to inches by dividing by 25.4. This is the standard P-metric conversion used by tire manufacturers and codified in TRA and ETRTO standards.
Why does tire diameter matter?
Overall diameter affects speedometer accuracy, odometer readings, gear ratios, and clearance. A larger-than-stock diameter makes the speedometer read low (actual speed is higher). A smaller diameter makes it read high. NHTSA recommends staying within 3% of the factory diameter.
What is revolutions per mile and why does it matter?
Revolutions per mile = 63,360 inches per mile divided by the tire circumference in inches. A stock tire is typically around 800 to 1,000 revolutions per mile. Changing rev/mile by more than 3% affects ABS, traction control, and transmission shift points.
What is the aspect ratio on a tire?
The aspect ratio is the sidewall height expressed as a percentage of the section width. A 45-series tire has a sidewall height equal to 45% of its 225 mm width, which is 101.25 mm or about 3.99 inches. Lower aspect ratios give a sportier look but a stiffer, less comfortable ride.
Official sources
- Tire and Rim Association (TRA): TRA Year Book (P-metric tire standards).
- NHTSA Tire Safety: nhtsa.gov/equipment/tires.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.