Bicycles are measured according to their frame size. Bike frame size in turn is defined by the length of the seat tube--the vertical part of the frame that supports the seat and seat post assembly. Choosing a bicycle with the proper fit makes riding more comfortable and you have better control. Individuals vary, but if you are between 5 feet 8 inches and 6 feet tall, an 18-inch bike is most likely a good fit. If you are shorter, try a 16-inch bike. For taller persons, a 20-inch or larger model may be best.
Locate the top of the seat tube. This is the point where the seat post enters or joins the bike frame. The seat post is the part that the bicycle seat is actually connected to.
Find the bottom bracket. The bottom bracket is a cylindrical component at the bottom of the seat tube. It's where the pedals are connected to the bike.
Measure the length of the seat tube from the top of the seat tube to the center of the bottom bracket. That measurement is the bicycle's frame size.
Convert your measurement if necessary. Some bikes are sized in millimeters and some in inches. There are 25.4 millimeters in one inch. To convert inches to millimeters, multiply the measured inches by 25.4. To change millimeters to inches, divide your measurement in millimeters by 25.4.
Bike frame size is a useful guideline, but it’s not the final answer to sizing a bike. Look for three things while sitting on the bike. When a pedal is at its lowest point, your foot should be flat on the pedal with your leg fully extended. When the pedal is at its highest, your knee should not come up to the horizontal. See if you can touch the ground with your toe when the bike is upright. If you can’t adjust the seat to a position meeting these three criteria, the bike is not the right frame size for you.