In the same way that shirts or dresses are sold in different sizes, bicycles also come in sizes that fit people of a certain size or body type. The frame size is a primary factor (relative to the size of the rider) in determining a comfortable and safe cycling experience.
Traditionally, bicycle frame sizes have been expressed either in inches or centimetres; however, as frame designs have become more streamlined and nonlinear, many bike manufacturers now indicate bike size with the terms "Small", "Medium", "Large" or "X-Large" (S/M/L/XL).
Locate the top of the seat tube (the top of the vertical tube where the seat post is inserted).
Measure from the top of the seat tube to the centre of the bottom bracket (the middle of the housing that holds the crank or pedal arms). The measurement in inches or centimetres from the top of the seat tube to the centre of the crank is the frame size. For example, if the measurement is 22 inches, the frame size of the bike is 22 inches.
Multiply the frame size in inches by 2.54 to convert to centimetres. A 22-inch frame is 55.88 centimetres; round up for a 56 centimetre frame size. To convert from centimetres to inches, multiply the centimetre figure by .39. For example, a 60 centimetre frame is the equivalent of a 23.4-inch frame size.
Record the frame size measurement in both inches and centimetres, since some manufacturers express frame size in inches, some in centimetres.
Most new bikes have the frame size indicated on a sticker on the front of the seat tube. The frame size may appear in inches, centimetres or as S/M/L/XL. When you list a bike for sale, post the frame size in inches and centimetres or S/M/L/XL. Novices often understand simpler explanations, such as "This bike, size Large, should fit riders 5 feet 10 to 6 feet 1." Before you shop for a bike, determine what size bike frame is appropriate for you, or ask a dealer. Remember that bike frame size is only one data point to help you decide what bike will fit best. Some riders have long legs and short torsos, or vice versa, so always test ride a bike for fit and comfort. Seats, handlebars and bar stems can also be adjusted to customise the fit. Ebicycles provides an excellent bike frame size and bike fit tool on their site: www.ebicycles.com/bicycle-tools/measure-frame