Step one: Stand barefoot with your back against a wall. That's the way to measure your inseam. Hold a book between your legs. It should be 1-inch wide. Measure the length in centimeters from the floor to the top edge of the book.
Step two: Define your measurement for a center-to-center (C-C size) measured road bicycle frame, which measures from the center of the top tube and to the center of the bottom bracket, by multiplying your inseam by .65.
Step three: Determine your measurement for a center-to-top (C-T size) measured road bicycle frame, which measures from the top of the top tube to the center of the bottom bracket, by multiplying your inseam by .67. This accounts for the extra height of the top of the seat tube.
Step four: Adjust your measurement by subtracting about 10 to 12 centimeters from your center-to-top road bicycle size and converting it to inches to figure out your center-to-top mountain bike size.
Step five: Since women have longer legs and shorter torsos than men, they should look for frames specially made for women, or the frames with shorter top tubes. Men with shorter torsos or who prefer an upright position should also think about the frames with shorter top tubes.
Step six: Divide your height with your inseam if you have surprisingly long or short limbs. If the outcome is more than 2.2, you'll need a longer reach or top tube. But, if it is less than 2.2, you'll need a shorter reach or top tube.
Step seven: Test ride a few different frame sizes, taking into consideration any top tube length calculated by the saddle post, to get a clear view of the size most suitable for you.