Swapping tires isn’t a problem for most drivers especially if the old and new tire sizes are the same.
The need to calculate your tire size may arise for instance, when you buy a new car and wonder if it can use your old tires.
Formula to calculate tire size.
R is the rim size in inches.
H is the height
W is the width in millimeters and it is the first number to appear.
Wheel rim size (R) is multiplied by 25.4 to obtain the wheel rim size in millimeters.
Find the circumference for a tire with the 245/45 R 18 label.
Therefore, the tire size is 2,129 mm.