Most dirt bike tires have a set of numbers that indicate the tire width, aspect ration (height) and rim diameter. Sometimes you might see just the tire width and diameter in inches (i.e. 2.50 - 10), and this is common with smaller tires.

Using a common 110/100-18 rear tire as an example; the first number, 110, indicates the width of the tire in millimeters. The second number,100, is the aspect ratio, which is the height of the sidewall in relation to the width of the tread (100% of 110mm). The last number, 18, is the rim diameter in inches. So this tire is 110mm wide by 110mm high or tall and fits an 18" rear rim.

Using a common 80/100-21 rear tire as an example; the first number, 80, indicates the width of the tire in millimeters. The second number,100, is the aspect ratio, which is the height of the sidewall in relation to the width of the tread (100% of 80mm). The last number, 18, is the rim diameter in inches. So this tire is 80mm wide by 80mm high or tall and fits a 21" front rim.