Following are some simple guidelines for figuring out the proper frame size to accommodate your unique body geometry. There are lots of other factors, like bottom bracket (BB) width, seat height and handle bar width that go into proper fitting, but if you can match the proper frame size to your body, you should be able to achieve a comfortable and enjoyable ride.
To determine your correct frame size, you will need your actual inseam length. This is a measurement for bike sizing purposes and will likely be different than your pants length. For this measurement, stand in stocking feet with your back against a wall. With your feet about eight inches apart (pedal width), place a carpenter’s square (a notebook also works) tight against your crotch. It should be tight enough to simulate sitting on a bike. The other plane of the square should be against the wall. With a yardstick, measure from the floor to the top of carpenter’s square or notebook. Repeat several times and average the results. If you measured in inches, convert to centimeters by multiplying by 2.54.
To find your frame size, multiply your average inseam in centimeters by .67. For example if your inseam is 33 inches, multiply 33″ x 2.54= 84cm x .67= 56.28 cm. The frame size for a 33 inch inseam would be 56 centimeters.