For bicycle sizing, three main measurements are required. These are the seat tube and length of the top tubes and the angle of the seat tube. This angle is usually influenced by the body size. The length of your arms and legs are used to deicide on the most comfortable seating position.

For the tube length, usually the horizontal distance between centerlines of the head and the seat tube is taken. The straight line distance between the junction of seat tubes and the seat and centers of the bottom bracket is taken as the seat tube length. The seat tube length should be 2/3 rd of the inside leg measurement. This measurement is taken in an erect position. You should not be wearing any shoes and you feet should be separated by ten inches. This measures the vertical length from the floor to the crotch which is the lowermost part of your pelvic bone.

Top tube length is comparable to the length of your arms as well as your trunk. Length from the seat to the tip of your shoulder is measured to get the trunk length. For measuring forearm length, the back of the elbow is pushed to the center of your fish which is clenched. It is pushed against some upright support like a wall.

For these measurements, you need to sit with your back pushed against some vertical surface. Stretch you arms in front for measuring arm length. The length from the vertical support to the middle of your clasped fist is measured to get the arm length. Tube length is then calculated by using these measurements. There is a standard formula for calculating tube length.

Next, you need to measure the seat tube angle. For this you should measure the length of your lower legs and your thighs. Measure the horizontal distance between the vertical support and front of your knee cap to get the length of your thigh. Measure the distance from the ground to the top of your knee cap to get the measurements of your lower leg.

However, you should always remember that there is no formula which gives the appropriate riding position and frame size for each and every rider. So you can decide about your riding position only after you have had a number of rides. The seat angle is usually 75 degrees for riders who are tall and have long legs. However these riders should make some adjustments to the seat angle since they are prone to develop problems of back.

This bicycle sizing is normally done by fitters who work at the bicycle shop. They take the measurements accurately and make your bicycle perfect for use.