There are two ways that tire manufacturers display tire size, inches and metric. To find the tire size meaning in inches is very straight forward.
Example A : 33X12.50R15
The first number is your overall diameter of the tire. In this example the tire diameter or "height" is 33 inches tall. The second number represents the width of the tire. In most cases with inch measurement tires the width will have a .50 decimal after the initial number. In example "A" this reads 12.50, meaning the tire width is 12 and 1/2 inches. The final number simply is the wheel size or "diameter"of the wheel that the tire will fit on.
Finding the tire size meaning in metric measurements is a little more difficult and usually take a calculator to get exactly right.
Example B : 305/70R17
On metric tires the first number is your width of the tire in millimeters. In example "B" this reads 305mm. There are 25.4 mm in one inch. To convert 305mm to inches you divide 305mm by 25.4 to get 12". So your width of a 305mm tire is 12 inches. The next number represents the percentage of the width (305mm or 12") that is the sidewall measurement of the tire. In this example the side wall measurement of the tire would be 70% of 12" which is 8.41" or 214mm. To get the overall diameter or "height" of the tire you would add the two sidewalls and the diameter of the wheel. In example "B" the last number "17" is the wheel size the tire will fit on. Therefore the sidewall twice would be 8.41 x 2 = 16.82 plus the wheel "17" makes the tire height 16.82 + 17 = 33.82. So a 305/70R17 metric tire would calculate to an inch measurement tire of 33.82x12R17. Remember, not all tire sizes are manufactured in both metric and inch sizes so learning the tire size meaning can be a valuable tool for anyone in the market for new tires.