The size of pipes, fittings, flanges and valves are often given in inches as NPS - Nominal Pipe Size, or in metric units as DN - 'Diametre Nominal'.
Pipes are made of a wide variety of materials - like galvanized steel, black steel, copper, cast iron, concrete, and various plastics such as ABS, PVC, CPVC, polyethylene, polybutylene and more.
Pipes are identified by "nominal" or "trade" names that are loosely related to the actual dimensions. For instance, a 2-inch galvanized steel pipe has an inside diameter of about 2 1/8 inches and an outside diameter of about 2 5/8 inches.
In plumbing pipe size is referred to as nominal pipe size - NPS, or "Nominal Pipe Size". The metric equivalent is called DN or "diametre nominel". The metric designations conform to International Standards Organization (ISO) usage and apply to all plumbing, natural gas, heating oil, and miscellaneous piping used in buildings. The use of NPS does not conform to American Standard pipe designations where the term NPS means "National Pipe Thread Straight".
ISO 6708 - Pipework components - Definition and selection of DN (nominal size)
ISO 6708 defines the nominal size - DN - as an alphanumeric designation of size for reference purposes. It comprises the letters DN followed by a dimensionless whole number which is indirectly related to the physical size in millimetres of the bore (ID) or outside diameter (OD) of the end connections.
Outside diameters for metric and imperial standards are indicated in the table below.
|Nominal Pipe Size - NPS||Outside Diameter (mm)|
|DIN EN 10220|
Seamless Steel Pipes
|DIN EN 10255 |
- NPS - Nominal Pipe Size - references inside diameter of pipe
- IPS - Iron Pipe Size - initially a system established to designate the pipe size representing the approximate inside diameter of the pipe
- DIPS - Ductile Iron Pipe Size - references inside diameter of pipe
- CTS - Copper Tube Size - references outside diameter of tube