This is a number system setup by ANSI and ASME for the purpose of standardization to designate the size of a pipe. Pipe sizes are normally referred to as "nominal pipe size" (NPS). For pipe sizes from 1/8" thru 12" (nominal inside diameter), 14" and larger (actual outside diameter). The larger the schedule number the larger the pipe size. The 1/8" thru 12" diameters differ from the actual inside diameter.