Nominal Diameter (DN) o Nominal Pipe Size (NPS)


The nominal diameter is the main concept that guides the definition a piping line. It is not the purpose of this guide to explain the standards and regulations, but a brief explanation may be helpful.

The Nominal Pipe Size is a standardized system used to designate the size of pipes the field of piping and plumbing. This is a nominal designation rather than an actual measurement of the outside diameter of the pipe. NPS is used mainly in North America while other part of the world could use different systems such as the Nominal Diameter (DN) in Europe.

Here are some key points about Nominal Pipe Size:

  • Nominal Dimension: NPS does not represent the actual measurement of the outside diameter. Rather, it is a nominal size used for identification purposes. For example, a pipe marked as a NPS 4 does not necessarily have an outside diameter of 4 inches.
  • Standardization: NPS is standardized by organizations such as the American National Standard Institute (ANSI) and the American Society for Testing and Material (ASTM). These standards ensure uniformity and compatibility in the production and use of pipes.
  • Relationship to Actual Diameter: The actual outside diameter of a pipe marked with a NPS size may very depending on wall thickness and material. For example, a pipe marked as NPS 4 could have a slightly different outside diameter than another pipe marked as NPS 4 but with a different schedule (wall thickness)
  • Schedule: In addition to NPS, pipes are often classified by their schedule, which indicated wall thickness. Common schedules include Schedule 10, Schedule 40 and Schedule 80.
    The schedule value affects the inside diameter, pressure rating and weight of the pipe.

Understanding the NPS concept is crucial to select the correct pipes and fittings for a piping system. Correctly matching tubing sizes ensures compatibility and efficient flow of fluid within the system.

In summary, nominal pipe size is a standardized system used to designate the size of pipes, providing a language for communication in the piping industry. Helps ensure consistency and compatibility in the design, manufacturing and installation of piping systems.