A tire calculator can be used to determine the height, sidewall thickness, circumference and revolutions per mile of any tire by typing the tire's size rating in. The size rating is a number following the format 285/75R16 found on the tire's sidewall.
The tire size rating gives only a tire's width, aspect ratio and the diameter of wheel it fits. The aspect ratio of a tire is defined as the ratio of the height of the tire to its width. Thus, a higher aspect ratio yields a larger sidewall, or a taller tire.
A tire size calculator can be used to find tires with similar specifications to your current tires. For example, if you want a tire with a lower sidewall but a width and wheel diameter equal to your current tire, you can break down your tire's information into the relevant numbers.
The revolutions per mile calculation is useful if you are buying tires that have a circumference larger or smaller than your current tire. As your vehicle's speedometer is calibrated to a particular tire size, using differently sized tires can throw it off. Some calculator's also include a speedometer adjustment ratio, which allows you to calculate the degree to which your speedometer will be off given a particular tire circumference.