For those simply interested in the formula itself, it is simple enough. First, the rider must measure his inseam. It is not recommended that the pants size be used as this may not be accurate enough, an inch one way or the other makes a pretty big difference. Stand barefoot on a hard floor, straight, feet fairly close together, back against a wall, and have an assistant insert a clipboard between your legs. Lift gently, sliding the clipboard as high as it will comfortably go while keeping it firmly against the wall, thereby assuring it isn't tilted. While holding the clipboard this way, measure the distance from the top edge of the clipboard to the floor, in inches. Don't round to the nearest inch; measure as accurately as possible.
L(mm) = 5.48 x I(in)
Now, multiply the inseam measurement (in inches) by 5.48. This provides a good estimate of proper crank length, in millimeters, for general road cycling or racing.