Metric to Inch Tire Size Conversion

If you look on the side of your car or truck's tire, you'll see a series of numbers, usually designated like the following example: P185/60R14. The first of those numbers is the tire width in millimeters. The second is the aspect ratio, which measures the width of the sidewall in relationship to the overall width. The third number is the wheel size, measured in inches. All of these numbers can be expressed in terms of standard values.

Instructions

1

Write down the tire size as found on the sidewall of your vehicle's tire. The number is located near the outer edge of the tire.

2

Multiply the first number, the tire width, by .03937. This will convert the the width from millimeters to inches. Using the above example of 185 millimeters, the tire would have a width of 7.28 inches.

3

Multiply the tire width (the number found in Step 2) by the percentage found in the aspect ratio, which is the second of the three numbers in the tire size. Using the example tire, the sidewall is 60 percent of the width, or 4.37 inches.

4

Multiply the sidewall width (the number found in Step 3) by two. This is necessary in determining the wheel's diameter because the sidewall is featured on both sides of the tire rim. The example tire would calculate to 8.74 inches of sidewall.

5

Add the third number found on the tire, the wheel size, to the total sidewall width (the number that was found in Step 4). This total will provide the complete tire diameter, including both the tire and wheel assembly. The P185/60R14 will have a diameter of 22.74 inches.