The ISO measuring system uses millimeters to signify the size of your bicycle's wheel. If you don't have a measuring tape with metric rules, just be aware that there are 25.4 millimeters in one inch and determine the width with a calculator, multiplying inches x 25.4.
 Measure in millimeters from the center of the wheel to only the inner edge of the tire. Once again, double the figure to determine the diameter. Most adult bicycle tires measured with the ISO system range from 650 to 700 millimeters in diameter.
 Use the same process listed above, across the tread from one side to the other. Keep in mind that tires of various widths can be used on the same bicycle wheel, as long as the difference is not too drastic.
 For example, a bicycle wheel sized 53.3 x 700 would have tread 53.3 millimeters wide and a wheel with a 700-millimeter diameter from the inner edge of one side of the tire to the other.
Recognize if your bike's wheels are measured by the International Organization for Standardization system instead of the standard method.
Ensure the bike is propped up against the wall or on its kickstand.
Measure the tire's width in millimeters.
Remember when purchasing a new tire measured by the ISO system, the width is listed first, followed by the diameter.
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