FRAME SIZE
The size of the frame is nearly always indicates the frame height expressed in centimeters. Sometimes you see a chart that show millimeters. Some companies offer a metric and an English chart.
There are various ways to establish the exact frame height; please note: the calculation of the frame height can differ per bicycle manufacturer and per country! Please ask your dealer to inform you about the measuring methods of the various bicycle manufacturers. Listed below are the three most common ways to calculate the height of a frame.
Centre to centre
Here the frame height is determined by measuring the distance from the centre of the bracket to the centre of the seat lug (the point at which the center line of the seat tube and the center line of the top tube cross).

Centre to top
With this method the height is established by measuring the distance from the centre of the bracket to the top side of the seat lug. As a rule, this frame height equals the centre-to-centre height, plus 15 to 20 millimeters.

Top to top
This method of measuring the frame height is often applied for reasons of convenience because it is not always easy to determine the exact centre of the bracket. It is the distance from the top side of the pedal axle to the top side of the seat lug. It is safe to say that the top-top height equals the centre-centre height.
