Users started to call the pipe as 2inch, 4inch, 6inch pipe and so on.
An IPS 6' pipe is one whose inside diameter is approximately 6 inches. The size represented the approximate inside diameter of the pipe in inches. That IPS system was established to designate the pipe size. The name NPS is based on the earlier Iron Pipe Size (IPS) system. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) for diameter based on inches, and a schedule (Sched. Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes and fittings used for high or low pressures and temperatures.
