Nominal Pipe Size

What Does Nominal Pipe Size Mean?

The nominal pipe size (NPS) is best thought of as "approximate" pipe size and refers to the bore of the pipe.

The outside diameter of plastic pipe always remains the same, but, as the wall thickness changes for different pressure ratings, the bore of the pipe varies. For example, 1" pipe has an outside diameter of 1.315 inches (33.4mm) and a bore of "approximately 1 inch". This sizing regime is a descendant from the Iron Pipe Size (IPS) system used in the early 1900s. The term "nominal bore" is often used in place of nominal pipe size.

Why Is My Plastic Pipe Not the Size I Ordered?

In Europe and some other parts of the world, metric pipe specifications denote the exact outside diameter of the pipe. In North America, the schedule system denotes the nominal pipe size, which approximates the internal diameter of the pipe.

Therefore, there is often confusion when ordering the different specifications of pipe, as the way the dimensions are represented are not consistent.

TABLE OF NOMINAL BORES

Are Metric PVC Pipes Measured by a Nominal Pipe Size?

Metric pipes are not measured using the nominal pipe size (NPS) system. The NPS system is used for North American inch sizes of pipe only and represents the approximate measurement of the internal diameter.

Metric pipes are measured using a system that relates to the outside diameter of the pipe, with a quoted nominal diameter (ND or DN), which represents the approximate bore.

Is DN the Same as Nominal Pipe Size?

DN (diametre nominale or nominal diameter) represents the approximate bore of the pipe and is used in metric pipe systems as a secondary classification after the pipe size, which denotes the outside diameter of the pipe. This is not the same as the nominal pipe size, which also relates to the approximate bore but is used as the main classification for North American pipes.

3 Things You Should Know About Nominal Pipe Size

1. Nominal pipe size is a pipe measurement system that refers to the North American "schedule system" of pipe classification.
2. The nominal pipe size refers to the approximate bore (internal diameter) of the pipe.
3. Metric pipes are not covered by the nominal pipe size specification.