As the late bicycling expert Sheldon Brown's website notes, with so many bicycle sizes and nuances in carbon frame geometry, determining the size of a carbon frame is no longer an exact science. Generally speaking, though, a tried-and-true old school method most often yields the correct frame size number. Being able to determine frame size is crucial in numerous situations, particularly if you are in the market for a used bike that does not bare a label indicating its size.
Lean your bicycle, or the bicycle in question, up against a wall or other sturdy surface. Locate the bottom bracket portion of the frame as well as the seat tube. The bottom bracket is the part of the frame that the crank arms fit into. The crank arms are the parts, on either side of the bottom bracket, that accept your pedals. The seat tube extends from the bottom bracket up to the part of the frame where you insert the seatpost. The seatpost goes into the top of the seat tube.
Measure from the center portion of the bottom bracket, along the seat tube to the top of the seat tube. You can use any type of tape measure to take this measurement.
Record your measurement in either inches or centimeters. Typically, mountain bike frames are measured in inches, ranging in size from extra-small (13-inches) to extra-extra-large (23 inches). Mountain bike sizes usually advance in 2-inch increments. Generally, you figure road bike frame sizes in centimeters. While variation is common from company to company, a typical size run ranges from extra-small (48-centimeters) to extra-extra-large (63-centimeters).
Tips & Warnings
Frame size is not always the most important number. Consider other aspects of frame geometry. For instance, top tube length measures the distance of the part of the carbon frame that extends from the seat tube to the head tube (where the fork and handlebar meets). This measurement is crucial when evaluating your horizontal fit on a bike.