To calculate a good bike size fit, taking some body measurements followed by some basic formulas using the measured inseam, torso and arm length is the starting place. The basic measurements of leg inseam, which measures the distance from crotch to foot, torso length, which measures from the crotch to the sternum, and arm length, which measures from the end of the collarbone to the middle of a closed fist, are necessary.
The inseam is most commonly consulted for finding good bike fit. Most bike size charts reference a stand over height, which is the inseam measurement plus one to two inches for clearance of the top tube. Some sources state the a one to two-inch clearance is needed for a road bike, while a mountain or commuter bike requires two to four inches.
Measuring the torso helps to determine an appropriate top tube length, the distance from the saddle to the handlebars, on a bike. To determine a good tube length, a good formula is adding the torso length and the arm length, dividing that number by two, and then subtracting six. The result is the ideal length in inches between the seat and handlebars.
Charts are also available to help determine a good bike size after taking body measurements.