The tyre size is indicated on the sidewall of a tyre in the form of a sequence of characters, e.g. 255/55 R18, where the first number is the cross section width measured between the tyre sides in millimetres.
The second number in tyre size is the height of profile (aspect ratio), i.e. a percentage to tyre width, e.g. 55 is 55% of 205, i.e. 112.75 mm. ”R” indicates the tyre type (R - radial, D - diagonal).
The last number denotes the width of a rim in inches. Knowing the tyre size is essential when making the purchase.