To use the Tire Size Calculator, you need to find the tire dimensions located on the sidewall of your tire, or in your vehicle owner's manual. These dimensions include the tire width (the width of the tire from sidewall to sidewall), the aspect ratio (the height of the tire's sidewall as a percentage of the width), and the wheel diameter (the diameter of the wheel the tire fits). They are typically displayed in a format like "265/70R17".

The formula for the tire size calculator computes various metrics of a tire based on the tire width, aspect ratio, and diameter. First, the sidewall height is calculated by multiplying the tire width by the aspect ratio and then dividing by 100. The overall diameter of the tire is determined by adding twice the sidewall height to the diameter converted to millimeters. The circumference is then calculated by multiplying the overall diameter by π. Finally, the revolutions per mile and per kilometer are calculated by dividing 63,360 by the circumference in inches and 1,000,000 by the circumference in millimeters, respectively.

These calculations help ensure your tire's compatibility with your vehicle and aid in making informed decisions when purchasing new tires.