Bicycle tire measurements have two components. The larger number is the tire diameter in inches, and the smaller number is the tire width in inches. Tire diameter usually ranges from 12 to 26 inches, and tire width normally ranges from 1.75 to 2.215 inches. The International Organization for Standardization (ISO) has developed another bicycle tire measurement system that uses millimeters to measure the width of the tire and the inner diameter, also called the bead seat diameter. Replacing tires may be difficult if you know the size in one system, but only find tires measured in the other system. Although most tires have the measurements imprinted on the tire, the numbers can become worn and unreadable over time.
Stand up the bicycle with the kickstand, or lean it up against a sturdy wall.
Hold the end of the tape measure against the center of the bicycle wheel, and extend the tape in a straight line to the outer edge of the tire. For traditional sizing, double the inches to find the bike tire diameter. To determine the ISO diameter, measure in millimeters from the center of the wheel to only the inner edge of the tire and double the figure.
Measure the flat surface across the tire's tread from one side of the tire to the other. This is the tire width. Measure in inches for traditional tire measurements or in millimeters for the ISO measurement.
Combine the measurements of tire diameter and width to obtain the tire size. Traditional bike tire sizes place the diameter first and the width second. ISO sizes place the width first and the diameter second.