For most types of bicycle, you'll need to know your leg length or bicycle inseam to find the proper frame size. Measure your leg length by placing a thin, hard cover book between your legs with the spine firmly against your crotch. Have a helper measure the distance from the book spine to the floor. Measure inseam or leg length in centimeters if you are looking for a road bike or in inches for a mountain, comfort or children's bike.
Road Bikes
The frame size of a road bike measures the length of the seat tube, the part of the bike that holds the seat post. Road bike sizes are generally measured in centimeters, and they range in size from 46 to 63. Estimate your road bike size by subtracting 23 to 25 centimeters from your leg length. If you like to sit upright, first try the larger frame size in your range. Check for an appropriate fit by straddling the bike. You should be able to stand with your feet flat on the ground without touching the crossbar.
Mountain Bikes
Mountain bike sizes typically range from 14 to 24 inches and are also based on the seat tube length. Select the appropriate mountain bike based on the distance from the bike's top tube to the ground. This distance should be 2 to 5 inches shorter than your inseam length. Clearance of 4 to 5 inches allows for aggressive mountain biking, while 2 inches is fine for commuters, according to Sierra Trading Post.
Comfort/Cruiser Bikes
Comfort and cruiser bikes are intended for light leisure riding in a more upright position and don't require a custom fit. Rather than having sizes based on seat tube length, they simply come in small, medium, large and extra large sizes. A small comfort bike is for people with inseams between 27 and 29 inches, a medium is for inseams between 29 to 32, a large is for inseams between 32 to 34 and an extra large is for inseams between 34 to 37.
Children's Bikes
Children's bike sizes are based on wheel size, in inches. They usually come in sizes between 12 and 24 inches. When fitting a child for a bike, first try a bike size that is the same size or one size larger than the child's inseam. Have the child straddle the bike to make sure he doesn't touch the crossbar. Also check the handlebars, which should be no higher than the child's shoulders.
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