eg 215/55/15
215 is the width of the tire in mm's
the 55 is the aspect ratio of the tire.
Sample calculation
215MM X .55 = 118.25MM (section width from top of rim to top of tire) 118.25MM X 2 = 236.5MM (because there are two sections for the tire) + the diametre of the rim 15"= 381MM
Total size of tire: 617.5mm, in inches 24.31"
with different aspect ratios the section width will change. 55 series will have a smaller section width then say a 70 series.
to calculate mm from inches multipy inches by 25.4
to calculate inches from mm's divide by 25.4