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 centimetres, inches and as small, medium or large. It's handy to know how to convert between these sizing systems.
Convert centimetres to inches by dividing centimetres by 2.54. Round to one decimal place. For example a bike with a frame size stated as 52 centimetres (or just 52; the "centimeters" is implied) would be 20.5 inches (52/2.54=20.47).
Convert inches to centimetres by multiplying by 2.54. Round to the next whole number. For example an 18 inch bike equates to a frame size of 46 centimetres (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 centimetres (18 inches) and is best for someone 1.57 m to 1.68 m (5 feet, 2 inches to 5 feet, 6 inches) tall. A medium frame works best for someone 1.68 m to 1.8 m (5 feet, 6 inches to 5 feet, 11 inches) and averages 50 centimetres (19.7 inches). Large frames average 56 centimetres (22 inches) and are intended for people 1.8 m to 1.9 m (5 feet, 11 inches to 6 feet, 3 inches).
Your height is just one factor in determining the correct size of frame you need. Your local bike shop can measure your inseam and standover height to get you the best fit possible. Mountain bikes and commuter bikes are typically measured in inches, road bikes in centimetres.