A designation, rather than a direct measurement, indicates the approximate inside diameter of a pipe. It’s a standard number used to refer to the pipe size in all documentation, ordering, and fitting. For instance, a 2-inch designation doesn’t mean the internal diameter is precisely two inches; the actual inside diameter will vary based on the wall thickness, which depends on the pipe’s schedule or pressure rating. A pipe with a greater wall thickness has a smaller actual internal diameter despite sharing the same designation.
This sizing system simplifies specifying and identifying pipes across different manufacturers and materials. Historically, it developed as a convenient way to categorize pipe sizes before standardized wall thicknesses and manufacturing processes were widely adopted. The enduring value lies in facilitating consistent communication within the engineering and construction industries, preventing confusion that might arise from using exact internal measurements, which fluctuate with different wall thicknesses.
