Tire Size Calculations

Tire Size Calculations

If we asked you to tell us your shoe size or your pant size, you could likely provide the answer in a matter of seconds. If we asked you to tell us your tire size, it may take you slightly longer to provide the answer. The reason for this is because tire sizes are a mouthful. They are a combination of millimeters, percentages and inches, and they include a few slashes and letters just for good measure.

Example Size – P225/45R17

The first letter of the tire size (P225/45R17) indicates the tire class.  The letter “P” is used to identify Passenger Car Tires, and the letters “LT” are used to identify Light Truck Tires.

The first three digits (P225/45R17) is the tire’s section width.  This is the measurement in millimeters from the widest point of the inner sidewall to the widest point of the outer sidewall 


The second pair of numbers (225/45R17) is the tire's aspect ratio or profile. This number is expressed as a percentage, and tells us the relationship between the sidewall's section height as compared to its’ section width. For those with a mathematical mind, you can calculate the section height by multiplying the section width by the aspect ratio. The answer will be the height of one sidewall.


The second letter in the tire size (P225/45R17) indicates the tire’s internal construction.  This letter is a R, D or B.  In our example, the R means that the tire has radial construction. R stands for radial construction, D stands for diagonal construction and B stands for belted construction. Over 98% of all tires sold today are radial tires


The last two numbers (225/45R17) is the diameter of the wheel/rim in inches.

The diameter can be converted into millimeters by multiplying it by 25.4. For example:

17" x 25.4 = 431.8mm