DN to NPS Conversion Chart
DN (Diametre Nominal) is the metric pipe size designation used in Europe and internationally per ISO 6708. NPS (Nominal Pipe Size) is the inch-based designation used in North America per ASME B36.10. Both are dimensionless labels that identify pipe size; neither directly equals the actual outside diameter or inside diameter.
When This Conversion Is Used
The DN-to-NPS conversion is required on international EPC projects where engineering documents use DN (European standard) and procurement sources use NPS (ASME standard), or vice versa. Both designations refer to the same actual pipe OD, so the conversion ensures dimensional compatibility across mixed-standard specifications.
DN to NPS Conversion Table
| DN | NPS | OD (mm) | OD (in) |
|---|---|---|---|
| 6 | 1/8 | 10.3 | 0.405 |
| 8 | 1/4 | 13.7 | 0.540 |
| 10 | 3/8 | 17.1 | 0.675 |
| 15 | 1/2 | 21.3 | 0.840 |
| 20 | 3/4 | 26.7 | 1.050 |
| 25 | 1 | 33.4 | 1.315 |
| 32 | 1-1/4 | 42.2 | 1.660 |
| 40 | 1-1/2 | 48.3 | 1.900 |
| 50 | 2 | 60.3 | 2.375 |
| 65 | 2-1/2 | 73.0 | 2.875 |
| 80 | 3 | 88.9 | 3.500 |
| 90 | 3-1/2 | 101.6 | 4.000 |
| 100 | 4 | 114.3 | 4.500 |
| 125 | 5 | 141.3 | 5.563 |
| 150 | 6 | 168.3 | 6.625 |
| 200 | 8 | 219.1 | 8.625 |
| 250 | 10 | 273.1 | 10.750 |
| 300 | 12 | 323.9 | 12.750 |
| 350 | 14 | 355.6 | 14.000 |
| 400 | 16 | 406.4 | 16.000 |
| 450 | 18 | 457.2 | 18.000 |
| 500 | 20 | 508.0 | 20.000 |
| 550 | 22 | 558.8 | 22.000 |
| 600 | 24 | 610.0 | 24.000 |
| 650 | 26 | 660.4 | 26.000 |
| 700 | 28 | 711.2 | 28.000 |
| 750 | 30 | 762.0 | 30.000 |
| 800 | 32 | 812.8 | 32.000 |
| 900 | 36 | 914.4 | 36.000 |
| 1000 | 40 | 1016.0 | 40.000 |
| 1050 | 42 | 1066.8 | 42.000 |
| 1100 | 44 | 1117.6 | 44.000 |
| 1200 | 48 | 1219.2 | 48.000 |
OD values per ASME B36.10. DN values per ISO 6708.
Key Points
DN is not millimeters: DN 50 does not mean 50 mm OD. The actual OD is 60.3 mm (NPS 2). DN is a rounded nominal value that approximates the bore size in millimeters for historical reasons.
NPS is not inches (for small sizes): NPS 2 does not mean 2 inches OD. The actual OD is 2.375 inches. From NPS 14 and above, NPS equals the actual OD in inches.
Same physical pipe: DN 100 and NPS 4 refer to the same pipe with an OD of 114.3 mm. The wall thickness and schedule are independent of whether DN or NPS is used.
European pipe sizes: European EN 10220 defines additional pipe ODs that do not have direct NPS equivalents. These are primarily used in structural and water pipeline applications.
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