Nominal Pipe Size

Published: December 31, 2017 | Last updated: July 5, 2023

What Does Nominal Pipe Size Mean?

Nominal pipe size (NPS) is a set of standard North American sizes to designate pipe diameter and thickness for pipes that are used for low and high pressures and temperatures. Nominal means something that is not specified, and in terms of pipe sizes, it refers to the diameter of the pipe hole with a dimensionless number.

NPS is used for calculating the required pipe strength and outer diameter (OD) required for installing pipelines using trenchless methods since these methods require pulling the pipes through boreholes.


Trenchlesspedia Explains Nominal Pipe Size

The size of a pipe is specified by using two dimensionless numbers i.e. an NPS for diameter and schedule for wall thickness. NPS can be stated to be similar to the inside diameter (ID) in inches for NPS 1/8 to 12. The NPS is considered equal to OD when the NPS is 14 or greater.

The wall thickness stays constant or increases while the OD increases with the increasing NPS, for a given schedule; however, the wall thickness increases and OD stays constant with the larger schedule for a given NPS.


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