Calculating Approximate Tire Dimensions

Most modern passenger car and light truck tires feature size designations that indicate the tire's dimensions in a combination of metric, mathematical and English systems. While this unusual combination of millimeters, percentages and inches is a byproduct of the evolution of global tire specifications, it also provides the ability to calculate/estimate  basic  tire dimensions.

Example size: 225/45R17

The first three numbers in a typical size (225/45R17) are the tire's indicated section width in millimeters, measured from sidewall to sidewall.
If you are familiar with measurements in inches, the section width in millimeters can be converted into inches by dividing it by 25.4. For example:

225mm / 25.4 = 8.86"

The second pair of numbers (225/45R17) is the tire's aspect ratio or profile. This is a ratio of sidewall height to section width. The section height's measurement can be calculated by multiplying the section width by the aspect ratio. The answer will be the height of one sidewall. For example:

225mm x 0.45 = 101.3mm
8.86" x 0.45 = 3.99"