Comparison · naming systems
NPS to DN — what each size name is worth in millimetres
DN (Diamètre Nominal, ISO 6708) and NPS (ASME) are two naming systems for the same family of steel pipe. They agree on which pipe, not onwhat you measure: DN is not the outer diameter, and for sizes up to DN 50 it is not even close to it.
| DN | NPS | OD (in) | OD (mm) | Other names for this OD |
|---|---|---|---|---|
| DN 6 | NPS 1/8 | 0.405 | 10.287 | stainless 1/8 |
| DN 8 | NPS 1/4 | 0.540 | 13.716 | stainless 1/4 |
| DN 10 | NPS 3/8 | 0.675 | 17.145 | stainless 3/8 |
| DN 15 | NPS 1/2 | 0.840 | 21.336 | PVC 1/2 · stainless 1/2 |
| DN 20 | NPS 3/4 | 1.050 | 26.67 | PVC 3/4 · stainless 3/4 |
| DN 25 | NPS 1 | 1.315 | 33.401 | PVC 1 · stainless 1 |
| DN 32 | NPS 1 1/4 | 1.660 | 42.164 | stainless 1 1/4 |
| DN 40 | NPS 1 1/2 | 1.900 | 48.26 | PVC 1 1/2 · stainless 1 1/2 |
| DN 50 | NPS 2 | 2.375 | 60.325 | PVC 2 · stainless 2 |
| DN 65 | NPS 2 1/2 | 2.875 | 73.025 | stainless 2 1/2 |
| DN 80 | NPS 3 | 3.500 | 88.9 | PVC 3 · stainless 3 |
| DN 100 | NPS 4 | 4.500 | 114.3 | PVC 4 · stainless 4 |
| DN 125 | NPS 5 | 5.563 | 141.3 | stainless 5 |
| DN 150 | NPS 6 | 6.625 | 168.275 | PVC 6 · stainless 6 |
| DN 200 | NPS 8 | 8.625 | 219.075 | PVC 8 · stainless 8 |
| DN 250 | NPS 10 | 10.750 | 273.05 | stainless 10 |
| DN 300 | NPS 12 | 12.750 | 323.85 | stainless 12 |
The trap in the middle of the table
From DN 6 to DN 50 the DN number is roughly the bore of the original iron pipe, so DN 50 (about 50 mm) is NPS 2 whose outside is 60.3 mm. From DN 65 upward the DN number tracks the outside more closely — DN 65 is NPS 2½ at 73.0 mm. Nothing in the name changes; the habit behind it does. That is why a DN-only order slips: specify NPS or OD, and the schedule.
Next
- Full ASME B36.10M chart — every schedule, in and mm
- Steel vs PVC · steel vs PEX/CTS · carbon vs stainless
- Inch ↔ millimetre for pipe ODs