The staff at your local bike shop can help you choose a perfect frame size.
Like a good pair of shoes, the best bike is the one that fits you. Calculating the right frame size is an essential step when purchasing a new bike. A correctly sized and fitted frame will reduce soreness in your knees, neck and lower back during a ride, while also improving your pedaling efficiency for more power and speed. No matter your riding habits, calculating the right frame size should be your first priority.
Getting a Leg Up
Your leg length is the main factor in calculating your bike frame size. In cycling, this leg length measurement is based on your pubic bone height, a measure of your inseam that runs from the bony protrusion behind your genitals to the base of your heel along the inside of your leg. You'll want to wear the shoes you intend to cycle with when getting your measurement. This measurement corresponds to a bicycle manufacturer's frame size chart.
Frame Size Match-Up
Road bikes and mountain bikes use different frame sizing systems. Road bikes use the metric system; the height of the seat tube corresponds with your pubic bone height in centimeters, and this measurement usually falls between 48 and 63 centimeters. A mountain bike uses the standard system, but in inches; these frames usually fall between 15 and 22 inches. Most mountain bike companies size their bikes according to a small, medium, large, and X-large category system, but you can check the measurements of these frames through a manufacturer to compare different brands.
On The Edge of Your Seat
Now that you've matched your inseam to the seat tube, you can set the seat height to determine if the top tube of your frame matches your proportions. Lean the bike up against something solid and raise the seat tube until your leg is just slightly bent at the knee; the crank should be pointed straight down. If your knee is more than slightly bent, your seat is too low, and you won't be able to properly check the size of the top tube of your frame.
Coming out on Top
Your top tube length determines how far you need to lean forward to reach the handlebars, and is a key step in sizing a bike frame. Lean forward and grab the handlebars, bending your elbows a bit so so they don't lock up. In this position you shouldn't be straining your lower back or neck, and your rear should be firmly planted on the widest part of your seat. If you have to reach farther than you're comfortable, you'll want to get a shorter stem to connect the frame and handlebars, or even a smaller frame size altogether.