DN vs NPS: Metric vs Imperial Pipe Sizes
DN (Diametre Nominal) and NPS (Nominal Pipe Size) are two parallel systems for designating pipe size. Both are dimensionless labels, not direct measurements. DN is the metric system used in Europe and internationally per ISO 6708 and EN standards. NPS is the North American system used in ASME, ASTM, and API standards.
The critical point: DN and NPS designate the same pipe. A DN 100 pipe and an NPS 4 pipe have identical outside diameters (114.3 mm / 4.500 inches). They are the same physical product with different labels.
When to Use Each
NPS is used when:
- The project follows ASME/ANSI standards (B31.1, B31.3, B31.4, B31.8)
- Pipe is specified per ASTM or API standards
- The project is in North America or follows US engineering practices
DN is used when:
- The project follows EN/ISO standards (EN 13480, EN 10217, EN 10216)
- Pipe is specified per European norms (EN 10220, EN 10255)
- The project is in Europe, Middle East, or follows international practices
Conversion Table
| NPS | DN | Actual OD (mm) | Actual OD (in) |
|---|---|---|---|
| 1/8 | 6 | 10.3 | 0.405 |
| 1/4 | 8 | 13.7 | 0.540 |
| 3/8 | 10 | 17.1 | 0.675 |
| 1/2 | 15 | 21.3 | 0.840 |
| 3/4 | 20 | 26.7 | 1.050 |
| 1 | 25 | 33.4 | 1.315 |
| 1-1/4 | 32 | 42.2 | 1.660 |
| 1-1/2 | 40 | 48.3 | 1.900 |
| 2 | 50 | 60.3 | 2.375 |
| 2-1/2 | 65 | 73.0 | 2.875 |
| 3 | 80 | 88.9 | 3.500 |
| 4 | 100 | 114.3 | 4.500 |
| 6 | 150 | 168.3 | 6.625 |
| 8 | 200 | 219.1 | 8.625 |
| 10 | 250 | 273.1 | 10.750 |
| 12 | 300 | 323.9 | 12.750 |
| 14 | 350 | 355.6 | 14.000 |
| 16 | 400 | 406.4 | 16.000 |
| 18 | 450 | 457.2 | 18.000 |
| 20 | 500 | 508.0 | 20.000 |
| 24 | 600 | 610.0 | 24.000 |
| 30 | 750 | 762.0 | 30.000 |
| 36 | 900 | 914.4 | 36.000 |
| 42 | 1050 | 1066.8 | 42.000 |
| 48 | 1200 | 1219.2 | 48.000 |
Why Neither Value Equals the OD
For small sizes (below NPS 14 / DN 350), neither NPS nor DN equals the actual outside diameter. NPS 2 pipe has an OD of 60.3 mm, not 2 inches (50.8 mm). DN 50 pipe has the same OD of 60.3 mm, not 50 mm.
This dates back to early pipe manufacturing when pipe sizes approximated the internal bore. The OD was standardized but the label was not corrected. From NPS 14 / DN 350 upward, NPS equals the OD in inches (NPS 14 = 14.000” OD = 355.6 mm).
EN Pipe Sizes vs ASME Pipe Sizes
While DN and NPS are interchangeable for most sizes, some European pipe standards (EN 10220) include additional DN sizes not found in ASME B36.10:
| DN (EN only) | OD (mm) | ASME Equivalent |
|---|---|---|
| DN 32 | 42.4 | NPS 1-1/4 (42.2 mm OD) |
| DN 125 | 139.7 | NPS 5 (141.3 mm OD) |
| DN 175 | 193.7 | No direct equivalent |
| DN 225 | 244.5 | No direct equivalent |
The slight OD differences for DN 32 and DN 125 can cause fit-up issues when mixing EN and ASME components. Verify OD compatibility when sourcing from different standard systems. For detailed EN vs ASME pipe size charts, consult the full reference tables.
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