Convert centimeters to inches by dividing centimeters by 2.54. Round to one decimal place. For example a bike with a frame size stated as 52 centimeters (or just 52; the "centimeters" is implied) would be 20.5 inches (52/2.54=20.47).
Convert inches to centimeters by multiplying by 2.54. Round to the next whole number. For example an 18-inch bike equates to a frame size of 46 centimeters (18 x 2.54 = 45.7).
Determine a frame size listed as small, medium or large by consulting the manufacturer's product information or website or measuring the bike yourself, if possible. In general, a small frame averages 46 centimeters and is best for someone 5 feet, 2 inches tall to 5 feet, 6 inches. A medium frame works best for someone 5 feet, 6 inches to 5 feet, 11 inches and averages 50 centimeters. Large frames average 56 centimeters and are intended for people 5 feet, 11 inches to 6 feet, 3 inches.