NPS and DN Pipe Size Conversion
General / GeneralDefinition
NPS (Nominal Pipe Size) is the North American standard designation for pipe and valve sizes, expressed in inches. DN (Diametre Nominal) is the international (ISO 6708) metric equivalent, expressed in millimeters. Both are dimensionless designators — they do not represent the actual measured outside or inside diameter of the pipe. The relationship between NPS and DN is straightforward: DN = NPS x 25, rounded. However, the actual pipe OD varies by schedule (wall thickness), and for NPS 14 and above, the NPS number does approximate the OD in inches, while for NPS 12 and below, it does not.
Working Principle
For NPS 1/8 through NPS 12, the NPS number is a nominal size only — the actual outside diameter is larger than the NPS number in inches. For example, NPS 2 pipe has an OD of 2.375 inches (60.3 mm), not 2.000 inches. The wall thickness is defined by the schedule number (SCH 40, 80, 160, etc.) or by a specific designation (SCH 5S, 10S, XS, XXS). The inside diameter equals the OD minus twice the wall thickness. For NPS 14 and above, the NPS number equals the OD in inches: NPS 14 = 14.000 inch OD. DN values are simply NPS x 25, so DN 50 = NPS 2, DN 100 = NPS 4, DN 200 = NPS 8, etc.
Types & Variants
| NPS | DN | Actual OD (in) | Actual OD (mm) | SCH 40 ID (in) | SCH 80 ID (in) |
|---|---|---|---|---|---|
| 1/2 | 15 | 0.840 | 21.3 | 0.622 | 0.546 |
| 3/4 | 20 | 1.050 | 26.7 | 0.824 | 0.742 |
| 1 | 25 | 1.315 | 33.4 | 1.049 | 0.957 |
| 2 | 50 | 2.375 | 60.3 | 2.067 | 1.939 |
| 4 | 100 | 4.500 | 114.3 | 4.026 | 3.826 |
| 6 | 150 | 6.625 | 168.3 | 6.065 | 5.761 |
| 8 | 200 | 8.625 | 219.1 | 7.981 | 7.625 |
| 10 | 250 | 10.750 | 273.0 | 10.020 | 9.562 |
| 12 | 300 | 12.750 | 323.8 | 11.376 | 10.750 |
| 14 | 350 | 14.000 | 355.6 | 12.750 | 12.500 |
| 16 | 400 | 16.000 | 406.4 | 14.500 | 14.000 |
| 20 | 500 | 20.000 | 508.0 | 18.500 | 17.938 |
| 24 | 600 | 24.000 | 609.6 | 22.500 | 21.562 |
Materials
NPS/DN applies to all pipe materials — carbon steel, stainless steel, alloy steel, copper, PVC, HDPE, and all others. The OD is the same for a given NPS regardless of material. The wall thickness (schedule) varies by material: for example, stainless steel thin-wall pipe uses SCH 5S and 10S, while carbon steel typically uses SCH 40, 80, or 160.
Applicable Standards
ASME B36.10M (Welded and Seamless Wrought Steel Pipe) defines OD and wall thickness for all NPS sizes and schedules in carbon and alloy steel. ASME B36.19M covers stainless steel pipe schedules. ISO 6708 defines DN designations. ASME B16.5 flanges, B16.9 fittings, and B16.10 valve dimensions are all based on NPS. API 5L (Line Pipe) uses NPS designation with metric equivalents.
Applications
NPS/DN designations appear on every pipe, valve, fitting, flange, and component specification in the piping industry. Engineers specify valve size as NPS or DN based on the required flow capacity and pipe schedule. Procurement uses NPS/DN to order components from catalogs. Piping design software uses NPS to select correct dimensions for routing and stress analysis. International projects often show both NPS and DN on the same drawings to avoid confusion between imperial and metric stakeholders.
Advantages
- Universal designation system understood worldwide
- NPS/DN directly relates to component dimensions in all standards
- Simple conversion: DN = NPS x 25 (approximate)
- OD is fixed for each NPS regardless of wall thickness — simplifies fitting selection
- Well-established in every piping code and standard globally
Disadvantages
- NPS does not equal actual OD for sizes NPS