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NPS - 'Nominal Pipe Size' and DN - 'Diametre Nominal'

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'

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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 - NPSOutside Diameter (mm)
DN
(mm)
InchISO 6708
Pipework components
DIN EN 10220
Seamless Steel Pipes
DIN EN 10255
Threaded Tube
ASME
10 3/8 17.2
15 1/2 21.3 20.0 21.3 21.3
20 3/4 26.9 25.0 26.9 26.7
25 1 33.7 30.0 33.7 33.4
32 1 1/4 42.4 38.0 42.4 42.2
40 1 1/2 48.3 44.5 48.3 48.3
50 2 60.3 57.0 60.3 60.3
- 2 1/2 - - 73.0 73.0
65 - 76.1 76.1 76.1 -
80 3 88.9 88.9 88.9 88.9
- 3 1/2 - - 101.6 101.6
100 4 114.3 108 114.3 114.3
125 - 139.7 133 139.7 -
- 5 - - 141.3 141.3
150 6 168.3 159 168.3 168.3
200 8 219.1 216 219.1 219.1
250 10 273.0 267 273.0 273.0
300 12 323.9 318 323.9 323.8
350 14 355.6 368 355.6 355.6
400 16 406.4 419 406.4 406.4
450 18 457 470 457 457
500 20 508 521 508 508
600 24 610 622 610 610
700 28 711 720 711 711
800 32 813 820 813 813
900 36 914 920 914 914
1000 40 1016 1020 1016 1016
1200 48 1220 1219 1219

  • 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

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