Whether you’re a recreational cyclist or a finely-tuned racing machine, getting the most enjoyment from your time on a bicycle starts with choosing a correctly sized frame. An appropriately sized bicycle ensures comfort on long rides, maximizes sprinting or hill-climbing potential, and assists in the prevention of injuries. The right fit is relatively easy to determine from a few key body measurements, putting a professional bike fit within easy grasp.
Sizing the bike frame
Stand barefoot against a wall, with your back straight and your feet about 6 inches apart. Pull the book or carpenter’s square between your legs with an edge against the wall—the pressure should be firm enough to simulate that of a bike saddle. Mark the top edge of the book or carpenter’s square, and measure the distance to the floor in centimeters—this is your inseam, and is the measurement that will most directly determine your frame size.
Determine whether the frame you’re interested in is measured center to top or center to center. Center to top frame sizes reflect the distance from the center of the bottom bracket to the top of the top tube or seat tube (whichever is higher). Center to center frame sizes reflect the distance from the center of the bottom bracket to the center of the top tube. Your bike dealer will be able to tell you how frame size is determined for a particular bike—or you can take a tape measure to the shop and verify the size for yourself.
Multiply your inseam by .67 (for frames measured center to top) or .65 (for frames measured center to center). For example, an inseam of 75cm corresponds to a center to top frame size of 50cm (75 x .67 = 50.2).
Use comfort, personal body geometry, and preference to determine the best top tube length. Different brands and models of road bikes have different top tube lengths. This length is not reflected in the frame size, but is nevertheless important to fit. Riders with longer torsos need slightly longer top tubes than riders with shorter torsos (including most women). It pays to try several different models of a given frame size to determine which feels most comfortable. Keep in mind that top tube length can be corrected somewhat by choosing the correct stem—shorter stems mitigate slightly over-long top tubes, and vice versa.
Tips and Warnings
- It may be easier to get an accurate inseam measurement with the aid of a helper.
Measurements in inches can be converted to centimeters by multiplying by 2.54.
Your bicycle dealer can tell you whether a particular frame is measured center to center or center to top—or you can measure it yourself.
Very tall cyclists (particularly those with very long torsos) may need frames slightly larger than suggested by the formula. Use comfort as a guide.
Most women have shorter torsos than men with comparable inseams. Female riders may need to look for bicycles with somewhat short top tubes or bicycles specifically designed for women.
About this Author
Kirstin Hendrickson is a writer, teacher, coach and athlete. She’s been teaching, coaching, and writing about health, wellness and nutrition for more than 10 years. She has a B.S. in zoology, a B.S. in psychology, an M.S. in chemistry, and a Ph.D. in bioorganic chemistry.