Tire Size Calculator
To use the Tire Calculator, enter the top part of the data into the form. For Example a tire size of P245-45R-17 would make you Enter 245 for the Width, 45 for the Aspect Ratio and 17 is the Rim Diameter. Other style of tires (truck, race, etc) may have a different measuring system but it is pretty easy to figure out.
Note that this is the CALCULATED way to determine the size. For exact, look for the tire specs that you can often find online. The TireRack is a good source of that information as well as the manufacturer of the tire.
The image to the right show most all the basic measurements on a tire. Note that some of the measurements are dependant on the rim width. For example if you are using a wider rim the Section Width might be less then on a smaller width rim. It's always best to get a rim and fit the tire to it for exact measurements.
About the only tricky measurement is the 'Aspect Ratio' It's pretty simple too -
It is simply the ratio of sidewall height to section width. This determines the 'Section Height' of the tire. For example if you have a 245-45-17 tire it should come out close to
245mm (Width) x 0.45 (Aspect) = 110.45mm or 4.34inches