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'

Pipe is 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

http://www.engineeringtoolbox.com/nps-nominal-pipe-sizes-d_45.html