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 percentage represents the ratio of the sidewall's section height to the tire's section width. The section height can be calculated by multiplying the section width by the aspect ratio percentage. The answer will be the height of one sidewall. For example:
225mm x 0.45 = 101.3mm
8.86" x 0.45 = 3.99"
The last number (225/45R17) is the diameter of the wheel in inches.
If you are familiar with measurements in the metric system, the wheel diameter can be converted into millimeters by multiplying it by 25.4. For example:
17" x 25.4 = 431.8mm
To calculate the overall diameter of a tire, the sidewall height must be multiplied by 2 (remembering that the tire diameter is made up of 2 sidewalls, the one above the wheel and the one below the wheel touching the ground) and add the diameter of the wheel.
101.3mm + 101.3mm + 431.8mm = 634.4mm
3.99" + 3.99" + 17" = 24.98"
"[The] unusual combination of millimeters, percentages and inches is a by product of the evolution of global tire specifications, [and] provides the ability to calculate/estimate basic tire dimensions."
Warning: These calculations (and those provided by all online tire dimension calculators) only reflect nominal tire dimensions, which are often subtly or substantially different than the tire's actual physical size and/or the dimensions provided by the tire manufactures.
Drivers working with close fitment tolerances or trying to maintain the original tire diameter of staggered size fitments (similarities or differences) should use the tire manufacturer's specifications for the exact tire(s) they are considering.
Actual Tire Section Width is Dependent on Wheel Width
All tire sizes are assigned specific rim widths upon which they are measured (measuring rim), as well as can often be mounted on slightly narrower or wider wheels (rim width range). Therefore, it is important to note that actual tire section width will depend on the wheel width the tire is mounted on. The rule of thumb is that tire section width changes by 0.2" for every 0.5" change in rim width, being reduced if mounted on narrower than measuring wheel and increased when mounted on wider wheels.