NPS (Nominal Pipe Size, inches) and DN (Diamètre Nominal, mm) describe the same pipe sizes in two systems. Neither is the actual pipe diameter – both are nominal references tied to standardized ODs. Use this table to convert between them.
NPS to DN Conversion Table
| NPS (in) | DN (mm) | Actual OD (mm) | Actual OD (in) |
|---|---|---|---|
| 1/2 | 15 | 21.3 | 0.840 |
| 3/4 | 20 | 26.7 | 1.050 |
| 1 | 25 | 33.4 | 1.315 |
| 2 | 50 | 60.3 | 2.375 |
| 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 | 273.0 | 10.750 |
| 12 | 300 | 323.9 | 12.750 |
| 14 | 350 | 355.6 | 14.000 |
| 16 | 400 | 406.4 | 16.000 |
| 20 | 500 | 508.0 | 20.000 |
| 24 | 600 | 610.0 | 24.000 |
How NPS Works
Below NPS 14″, the nominal size roughly tracks the inside diameter of standard-wall pipe – a “4 inch” pipe has 4.5″ OD (not 4″). From NPS 14″ upward, the nominal equals the OD in inches (14″ pipe = 14.0″ OD). DN follows the metric sequence (10, 15, 20, 25…), roughly the nominal bore in mm. This is why ordering by actual measured OD causes mistakes – always order by NPS/DN plus schedule.
Why It Matters When Buying Fittings
A DN100 flange bolts to a NPS 4″ flange – same drilling, same OD. But a flange ordered for “4 inch OD pipe” (114.3mm) can be confused with 100mm OD metric pipe (which is DN90, a non-standard size). State NPS or DN consistently on the PO, never raw OD, and confirm the standard (ASME vs DIN vs JIS) since drilling differs. We manufacture to both systems – send your spec in either unit.
FAQs
Is DN the same as the pipe bore? Approximately for standard wall, but not exactly – DN100 pipe has about 102mm bore in SCH40. DN is a nominal reference, like NPS.
What is NPS 14 and above? From 14″ up, NPS equals the actual OD in inches – NPS 24 pipe really is 24.0″ outside diameter.
More: Flange Types Guide · What Is a Flange? · Schedule Chart · NPS vs DN · All Articles