Tire Size Calculator
Enter the section width, aspect ratio, and rim diameter to calculate the diameter and circumference of the tire, along with the revolutions per mile.
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How to Calculate Tire Dimensions
Commonly used dimensions when working with tires are overall diameter, circumference, sidewall, and revolutions per mile. Tire specifications are generally composed of three numbers, the section width, aspect ratio, and rim diameter. Using these numbers the tire dimensions above can be found using some simple formulas.
Tires are generally specified in the form 245/40R18 where 245 is the section width in millimeters, 40 is the aspect ratio, and 18 is the rim diameter in inches.
Section width is the width of the tire from the inner sidewall to the outer sidewall. Section width is usually measured in millimeters. Aspect ratio is the ratio between the section width and the sidewall height. The rim diameter is usually measured in inches.
How to Calculate Sidewall Height
The sidewall height is determined by the section width and the aspect ratio. The formula to find the sidewall height is:
sidewall = section width × aspect ratio 100
To find the sidewall, simply enter the section width and aspect ratio into the formula and solve. The result will be the sidewall measurement in millimeters, use a conversion calculator to convert to inches.
For example, let’s find the sidewall of a 245/40R18 tire.
sidewall = 245mm × 40 100
sidewall = 245mm × .4
sidewall = 98mm
How to Calculate Tire Diameter
The diameter of a tire can be found using the sidewall dimension from the previous step. The formula to calculate diameter is:
diameter = (2 × sidewall) + rim diameter
The overall tire diameter is thus the sidewall times two, plus the rim diameter.
For example, let’s find the diameter of a 245/40R18 tire. Recall that the sidewall for this tire is 98mm.
Start by converting the sidewall to inches. 98mm is equal to 3.86 inches.
diameter = (2 × 3.86″) + 18″
diameter = 7.72″ + 18″
diameter = 25.72″
How to Calculate the Circumference
The circumference of a tire can be found using the diameter and a simple circumference formula:
circumference = diameter × π
The circumference is equal to the diameter times pi. Pi is equal to roughly 3.14.
For example, let’s find the circumference of the 245/40R18 tire. The diameter that was found above is 25.72″.
circumference = 25.72″ × 3.14
circumference = 80.8″
How to Calculate the Revolutions per Mile
Once you have the tire circumference it’s possible to find the revolutions per mile using this formula:
rev/mile = 63,360 ÷ circumference
Revolutions per mile is equal to the the number of inches in a mile divided by the circumference in inches.
For example, let’s find the revolutions per mile of the 245/40R18 tire. Remember that the circumference is 80.8″.
rev/mile = 63,360 ÷ 80.8″
rev/mile = 784.16