The designation of modern tire size provides enough data to calculate the height, width and diameter of any tire. The challenge in converting a tire size to usable measurements is that part of the size is in English measurements and part is in metric. Once you understand what the different numbers listed in a tire size represent, you will be able to calculate the dimensions of any tire.
Write down the tire size listed on the side of the tire. The tire size will be in this format: 275/65R18. The size may have a "P" or "LT" in front of the numbers, but those can be disregarded.
Calculated the width of the tire in inches. The first number listed--in the example: 275--is the section width of the tire in millimeters. The section width is the measurement at the widest part of the tire, about half-way between the rim and tread. Convert the millimeters to inches by dividing the section with by 25.4. For the example, 275 divided by 25.4 gives a width of 10.83 inches.
Calculate the height of the tire in millimeters and inches. The second number in the tire size is the aspect ratio and is the percentage of the height of the tire to the width. Multiply the section width times the aspect ratio to get the height from the rim to tread. In the example, 275 times 65 percent gives a height of 178.75 millimeters. Convert to inches by dividing by 25.4. This tire has a height of 7.04 inches.
Calculate the diameter of the tire by adding the tire height times two to the rim diameter. The last number in a tire size is the size of rim in inches the tire fits on. In this case, an 18-inch rim. The tire height is from the rim to the tread so it must be added twice to the rim diameter. In the example, 7.04 plus 7.04 plus 18 provides a tire diameter of 32.08 inches.
