Buying the correct size bike for your body is important if you want a comfortable ride. If your bike is too small you will feel awkward and cramped on the bike. A bike that is too large will leave you feeling out of control while riding. When discussing frame size, this is the measurement of the length of the seat tube -- the part of the frame that extends from the saddle to the anchor point for the pedals known as the bottom bracket. Frames are measured in centimeters, inches and as small, medium or large. It's handy to know how to convert between these sizing systems.
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.