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'
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".
| Diameter Nominal DN (mm) | Nominal Pipe Size NPS (inches) |
|---|---|
| 6 | 1/8 |
| 8 | 1/4 |
| 10 | 3/8 |
| 15 | 1/2 |
| 20 | 3/4 |
| 25 | 1 |
| 32 | 1 1/4 |
| 40 | 1 1/2 |
| 50 | 2 |
| 65 | 2 1/2 |
| 80 | 3 |
| 100 | 4 |
| 150 | 6 |
| 200 | 8 |
| 250 | 10 |
| 300 | 12 |
| 350 | 14 |
| 400 | 16 |
| 450 | 18 |
| 500 | 20 |
| 550 | 22 |
| 600 | 24 |
| 650 | 26 |
| 700 | 28 |
| 750 | 30 |
| 800 | 32 |
| 900 | 36 |
| 1000 | 40 |
| 1050 | 42 |
| 1100 | 44 |
| 1200 | 48 |
| 1300 | 52 |
| 1400 | 56 |
| 1500 | 60 |
| 1600 | 64 |
| 1700 | 68 |
| 1800 | 72 |
| 1900 | 76 |
| 2000 | 80 |
| 2200 | 88 |
IPS - Iron Pipe Size - initially a system established to designate the pipe size representing the approximate inside diameter of the pipe.
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