Checkout Dimensional Conversions tables
SIZES ARE EXPRESSED DIFFERENTLY ACCORDING TO THE VARIOUS STANDARDS OR USES.
MTB AND CITY TYRE SIZES ARE GIVEN IN INCHES. In this example: 26 x 1 1/8 x 1 1/2, the first number (26) is the overall tyre diameter. The second number (1 1/8) is the height of the tyre in inches and fractions of an inch. The third number (1 1/2) is the tyre width in inches and fractions of an inch.
FRENCH SIZES are used for Road tyres. In this example: 650 x 32A, the first number (650) is the overall diameter of the tyre in millimeters. The second number (32) is the tyre width in millimeters. The letter A indicates that the rim is 590 mm.
ETRTO (EUROPEAN TYRE AND RIM TECHNICAL ORGANIZATION) SIZES are given for all tyres. In this example: 32 x 590, the first number (32) is the tyre width in millimeters. The second number (590) is the tyre diameter in millimeters. This chart shows the conversions for all the sizes.