They were developed to help industry professionals standardize how pipe specifications are referenced internationally, which has helped reduce confusion among resource agents and end-users. The SDR ratio, sometimes referred to as OD / t, has a relatively simple formula:

The term "OD" refers to the average outside diameter of the pipe, while the term "MW" refers to the minimum wall thickness of the pipe.

The schedule number tells you how thick a pipe is. Schedule number definition (SCH) is the wall thickness of a pipe. Since the value itself has no quantities or units, it is represented only by a number.

It is the ratio of design pressure to allowable pressure of pipes. SCH is approximately 1000 times this ratio, which gives you a schedule number formula for future calculations. The Higher SCH values increase the wall size of the pipe and change the approximate nominal pipe size (NPS). ie, the approximate inside diameter of the pipe.