There’s a good reason bike fit is the most important consideration when purchasing a bicycle: With a properly sized bike you’re more likely to enjoy a comfortable ride, easy handling, efficient energy transfer and thus, quicker riding times. But getting the right fit can be a bit of a challenge. Because there’s no industry standard for bike sizes, and sizes may vary slightly from one brand to the next, it is often best to try out a bike before purchasing it.
You need to know your bicycle inseam, which is different from clothing inseam, to help you determine your ideal bike size. Stand with your back against the wall on a hard flat surface. Spread your feet 6 to 8 inches apart and place the spine of a book between your legs so it touches your pubic bone. Ask a helper to measure the distance from the book spine to the floor. For a road bike, measure in centimeters, for all other types of bikes, measure in inches. This is your bicycle inseam.
Road bike frame sizes are based on the length of the seat tube, the tube that holds the bike seat post. Road bike sizes, which are usually measured in centimeters, range from 46 to 63. Subtract 23 to 25 centimeters from your inseam to estimate your road bike size. If you prefer to sit upright while riding, try the largest frame size in your range first. Make sure your bike is not too big by straddling the bike with your feet flat on the floor. You should have at least an inch of space between you and the crossbar.
Mountain bike sizes, which usually range from 14 to 21 inches, are also based on seat tube height. But the distance from the center of the bike’s top tube to the ground, or the stand-over height, is the most important measurement when sizing a mountain bike. Select a mountain bike with a stand-over height of 3 to 5 inches less than your inseam. The more aggressive of a mountain biker you are, the greater you want the difference to be. Another way to find your mountain bike size is to convert your road bike size to inches and subtract 4 inches.
As with mountain bikes, you find your size for a commuter bike by paying attention to stand-over height. Generally, you should look for a bike with a stand-over height of 2 inches less than your inseam.
Comfort bikes are designed for light leisure riding. On a comfort bike, you’ll find yourself in more of an upright position compared to the other kinds of bikes. Comfort or cruiser bikes come in four basic sizes: small, for riders with inseams between 27 and 29 inches; medium, for riders with inseams of 29 to 32 inches; large, for inseams of 32 to 34 inches; and extra large, for inseams of 34 to 37 inches.