To determine what size bike is the best fit, consider frame size and seat height. According to Bicycling magazine, first measure the rider's inseam length in centimeters, and then multiply that number by 0.65. The resulting number is the correct frame size for a road bike. For a mountain bike, subtract 10 to 12 centimeters from the road bike number, and to get the seat height, multiply the inseam measurement by 0.833.
Another way to determine a correct bike fit is to measure the rider against the bike. Wear the shoes typically worn when biking, and then straddle the bike frame. The inseam, or the distance from the bottom of the foot to the crotch, should be at least 1 inch above the frame of the bike. Next, sit on the seat of the bike, and put one foot on a pedal.
Press down on the pedal with a foot until the pedal reaches its lowest position. The pedal should be able to be pressed all the way down with a slight bend to the knee. If the pedal cannot be pressed all the way down, the seat height is too high. If the knee is bent a lot, the seat height is too low. Unlike frame heights, seat heights can be adjusted, so make sure to adjust the seat before deciding if the bike is a good fit.