Motorcycle tires are rated by several different systems, but in all systems, the diameter of the rim is measured in inches. Interestingly enough, rim diamter is still expressed in inches, even when using the metric system, which expresses other figures in millimetres. This makes choosing a tire, based on a universal rim size, easier, but if you want to know the size in millimetres, you'll have to convert it.
Find the motorcycle's rim diameter, which is usually a number like 16,17 or 18, printed on the side of the motorcycle tire. As an example, in the tire rating "140/90H16," the 16 at the end is the rim diameter.
Multiply the rim diameter, measured in inches, by 25.4 to convert to millimetres. As an example, a 16-inch diameter tire would be:
16 * 25.4 = 406-millimetres (rounded)
Convert the width size, if necessary. In the "inches" system, the tire rating may look like "4.50 H 16 4PR." The first number is the width, expressed in inches. Again, multiply that number by 25.4 to convert to millimetres, such that the width would be:
4.50 * 25.4 = 114-millimetres