The pipe schedule is very commonly expressed using numbers. It is an indication of the pipe wall thickness.
Pipes of different nominal pipe sizes (NPS) are available with a range of different wall thicknesses. These different valve thicknesses are identified by assigning a pipe schedule to each standard wall thickness for a nominal pipe size.
The pipe schedule number approximately indicates the value of the ratio '1000P/S', where P is the design pressure in the pipe and S is the allowable stress in the pipe walls and both the quantities are expressed in psi (pounds per square inch). The design pressure corresponds to wall thickness of the pipe.
For higher design pressure, pipe schedule and hence wall thickness is higher. The allowable stress in the pipe wall corresponds to the pipe material. So roughly, for the same pipe material, higher design pressure means higher schedule and higher pipe wall thickness.
Normally for pipes, the outer diameter is approximately standardized for each nominal pipe size. Hence to increase the wall thickness for increasing pipe schedule, the inner diameter of the pipe has to be decreased.
EnggCyclopedia's standard piping dimensions calculator can be used to quickly calculate standard piping dimensions for various pipe sizes and pipe schedule numbers. These standard dimensions are based on ANSI standard for stainless steel pipes.