Nominal diameter, also called Nominal Pipe Size (NPS), is a standardized designation used to specify the size of pipes, tubing, and related components in various industries, including plumbing, construction, and industrial processes. It's important to note that the nominal diameter is not the actual physical measurement of the diameter, rather, it is a standardized designation that helps communicate the size of pipes and components in a consistent manner. NPS 1/8 to NPS 12 is a approximate represented number, not exactly the inside or outside dimension. NPS 14 and larger is equal to the outside diameter.
The nominal diameter is primarily used to classify pipes and components into different sizes, which makes it easier to select and work with various materials and fittings across different projects and industries. It's essentially a convenient way to refer to the size of a pipe without needing to provide the exact physical measurements.
In addition to nominal diameter, pipes are also often categorized by their schedule number, which indicates the wall thickness of the pipe. These schedules are standardized and correlate with specific ranges of wall thickness for different nominal diameters. It's important to remember that when working with pipes and components, the actual dimensions and specifications should be referred to from reliable sources, such as industry standards and specifications, to ensure accurate selection and compatibility for a given project.