Article
Buyers in North America and the Middle East usually order pipe in inches (NPS), while buyers in Europe and many other markets use millimetres (DN). Both describe the same pipes, but neither number is the actual diameter.
What NPS and DN mean
- NPS (Nominal Pipe Size) – an inch-based size designation from ASME B36.10M and B36.19M.
- DN (Diamètre Nominal) – a metric size designation used in ISO and EN standards.
Both are labels for a family of pipes with a fixed outside diameter. The wall thickness, given by schedule or in mm, changes the inside diameter, not the outside.
Conversion table
| NPS | DN | Actual OD (mm) |
|---|---|---|
| 1/2" | 15 | 21.3 |
| 3/4" | 20 | 26.7 |
| 1" | 25 | 33.4 |
| 1 1/2" | 40 | 48.3 |
| 2" | 50 | 60.3 |
| 3" | 80 | 88.9 |
| 4" | 100 | 114.3 |
| 6" | 150 | 168.3 |
| 8" | 200 | 219.1 |
| 10" | 250 | 273.0 |
| 12" | 300 | 323.8 |
| 16" | 400 | 406.4 |
| 24" | 600 | 609.6 |
Watch out for different OD series
Some European standards (for example parts of the DIN/EN system) use slightly different ODs for the same DN, such as 104 mm or 108 mm instead of 114.3 mm for DN 100. If your pipe, fittings and flanges come from different standards, check that the ODs actually match.
Best practice in enquiries
Write both: 4" (DN 100), OD 114.3 mm × 6.02 mm wall (SCH 40S). Giving the actual OD and wall thickness removes all doubt, whatever standard the buyer or supplier uses.
Frequently asked questions
Is DN 100 the same as 4 inch?
Yes, under ASME/ISO sizing DN 100 corresponds to NPS 4 with an OD of 114.3 mm.
Does schedule change the outside diameter?
No. The OD is fixed for each size; schedule changes wall thickness and therefore the inside diameter.
Why include actual OD in an enquiry?
Because some standards use different ODs for the same nominal size, and the actual OD removes ambiguity.
Key takeaways
- NPS (inches) and DN (mm) are nominal labels, not real diameters.
- OD is fixed by size; schedule changes the bore.
- Always give actual OD × wall in enquiries.
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