Formula
The sidewall is a percent of the width, then both sidewalls are added to the rim:
diameter inches = rim + 2 × (width mm × aspect / 100) / 25.4
An inch is 25.4 millimeters. Circumference is π times that diameter, and revolutions per mile are 63,360 inches divided by the circumference. The percent gap between two sizes, and road speed from wheel revolutions, are separate pages.
Assumptions
- The size is a metric passenger marking such as 225/45R17.
- The tire is round and at its nominal size, not grown by heat or wear.
- The rim diameter is in inches, as marked on the tire.
Worked example
A 225/45R17 tire has an overall diameter of 24.97 in.
Input definitions
- Section width
- The tire's width in millimeters, the first number in the size.
- Aspect ratio
- Sidewall height as a percent of section width. 45 means the sidewall is 45 percent of 225 mm.
Limitations
A mounted tire can measure differently from the nominal size. This page does not check load index, speed rating, or whether the tire clears the vehicle.
Questions
What does 225/45R17 mean?
The section is 225 mm wide, the sidewall is 45 percent of that width, and the rim is 17 inches.
Why is circumference on this page?
It is π times the diameter already calculated. A separate revolutions or circumference calculator would repeat that step.