NPS and Class appear on almost every flange specification, and they are often misread. NPS identifies pipe size. Class identifies the flange pressure designation.
What NPS means
NPS stands for Nominal Pipe Size, the North American designator for pipe size. It is a label, not a measured dimension. For small sizes the actual outside diameter is larger than the number suggests. From NPS 14 upward, the outside diameter equals the NPS in inches.
| NPS | DN (approx.) | Pipe outside diameter (in) | Pipe outside diameter (mm) |
|---|---|---|---|
| 1/2 | 15 | 0.840 | 21.3 |
| 1 | 25 | 1.315 | 33.4 |
| 2 | 50 | 2.375 | 60.3 |
| 4 | 100 | 4.500 | 114.3 |
| 6 | 150 | 6.625 | 168.3 |
| 8 | 200 | 8.625 | 219.1 |
| 12 | 300 | 12.750 | 323.9 |
| 24 | 600 | 24.000 | 609.6 |
DN is the metric designator. The table shows common equivalents. Pipe diameters here follow the ASME B36.10M and B36.19M tables. Wall thickness is separate and is set by schedule.
What Class means
Under ASME B16.5, flanges are designated Class 150, 300, 400, 600, 900, 1500 and 2500. The class sets dimensions and links to a pressure-temperature rating table for each material group. Class 150 does not mean 150 psi. The allowable pressure falls as temperature rises, and differs by material.
Copper nickel: the pressure basis for copper-nickel flanges should come from your governing code and project specification. Do not read it from carbon-steel tables. Availability by size and class depends on flange type, material and standard.
Why both numbers matter
- NPS must match the pipe and the mating flange
- Class must match the mating flange, including bolt circle and number of bolts
- Facing (RF, FF, RTJ) must also be compatible
Large sizes
ASME B16.5 covers NPS 1/2 to 24. Above NPS 24, ASME B16.47 applies. For details, see ASME B16.5 flanges explained.
What to put in an enquiry
State NPS, class, facing, flange type and alloy. For weld neck flanges add the pipe wall thickness or schedule. Request a quote with those details and we can respond faster.
FAQs
Is NPS the same as inside diameter?
No. NPS is a nominal label. Pipe outside diameter is fixed for each size, and inside diameter depends on wall thickness.
What is the DN of NPS 4?
DN 100 is the usual metric equivalent of NPS 4.