Riding a bicycle that's the right size for your body can make a big impact on your comfort and speed. A bike that's too short will be hard to peddle and will put strain on your knees and back; one that's too tall can put you at risk for a dangerous tumble. There's no substitute for actually test-riding a bicycle before you buy, but there is a simple way to estimate the size of the bike frame you should be shopping for.

Determining the right bike frame size for you

1. Determine what kind of bike you'd like to ride. In general, you'll want a shorter frame for a mountain bike than for a road bike, so your legs can generate more force on the peddles.

2. Measure your inseam. Stand barefoot, and have a friend measure the length of your leg from just under your crotch to the floor. Make the measurement in centimeters (cm) if possible; otherwise convert your inch measurement to centimeters by multiplying it by 2.54. If you're measuring for a mountain bike, do the measurement in inches.
 
3. Determine the correct road bike size. Most bike frames are measured from the center of the crank area to the top of the of the seat post. To find the right sized road bike frame, multiply your inseam measurement by .67. So if your inseam is 91 cm, you would look for a road bike frame of 61 cm.

4. Determine the correct mountain bike size. Take your inseam measurement in inches and subtract 7.5 inches. This will be about the right length for a mountain bike frame.

5. Go to the bike store and try out a bike frame at the size you've determined. The staff at the store should be able to advise you on other aspects of proper bicycle fitting, such as seat and handlebar hight. Don't buy a bike unless you can be sure you can ride it comfortably.