NPS to DN Conversion Chart
NPS (Nominal Pipe Size, inches) and DN (Diameter Nominal, metric) are two naming systems for the same set of pipe sizes. This chart gives the exact equivalence for steel pipe to ASME B36.10, together with the outside diameter each size actually has — because neither the NPS number nor the DN number is a measurement of anything.
DN is not a millimetre conversion of NPS. DN 50 is NPS 2, but 2 in is 50.8 mm and the pipe's outside diameter is 60.325 mm. DN values are rounded to convenient whole numbers, so treat DN as a label rather than a dimension.
Below NPS 14, the nominal size matches no physical dimension at all. It is a historical name inherited from 19th-century wrought-iron pipe, whose bore it once approximated. NPS 2 pipe is 60.325 mm outside, with a bore that depends entirely on its schedule.
From NPS 14 upward, NPS is the outside diameter exactly. NPS 16 is exactly 16.000 in (406.4 mm) outside diameter. The transition at NPS 14 catches people out constantly when they move between small-bore and large-bore drawings.
| NPS (in) | DN | OD (mm) | OD (in) | OD = NPS? |
|---|---|---|---|---|
| 1/8" | DN 6 | 10.287 | 0.405 | no |
| 1/4" | DN 8 | 13.716 | 0.540 | no |
| 3/8" | DN 10 | 17.145 | 0.675 | no |
| 1/2" | DN 15 | 21.336 | 0.840 | no |
| 3/4" | DN 20 | 26.67 | 1.050 | no |
| 1" | DN 25 | 33.401 | 1.315 | no |
| 1-1/4" | DN 32 | 42.164 | 1.660 | no |
| 1-1/2" | DN 40 | 48.26 | 1.900 | no |
| 2" | DN 50 | 60.325 | 2.375 | no |
| 2-1/2" | DN 65 | 73.025 | 2.875 | no |
| 3" | DN 80 | 88.9 | 3.500 | no |
| 3-1/2" | DN 90 | 101.6 | 4.000 | no |
| 4" | DN 100 | 114.3 | 4.500 | no |
| 5" | DN 125 | 141.3 | 5.563 | no |
| 6" | DN 150 | 168.275 | 6.625 | no |
| 8" | DN 200 | 219.075 | 8.625 | no |
| 10" | DN 250 | 273.05 | 10.750 | no |
| 12" | DN 300 | 323.85 | 12.750 | no |
| 14" | DN 350 | 355.6 | 14.000 | yes — exactly |
| 16" | DN 400 | 406.4 | 16.000 | yes — exactly |
| 18" | DN 450 | 457.2 | 18.000 | yes — exactly |
| 20" | DN 500 | 508 | 20.000 | yes — exactly |
| 24" | DN 600 | 609.6 | 24.000 | yes — exactly |
Outside diameters are the exact ASME B36.10 values (nominal inches × 25.4), from the same data as our pipe schedule tables. The outside diameter is fixed for a given nominal size — the schedule sets the wall thickness, and therefore the bore.
Published tables round these differently, so you will see NPS 2 quoted as 60.3, 60.32, 60.325 or 60.33 mm. They are all the same pipe: ASME B36.10 fixes the outside diameter in inches, and B36.10M additionally defines a rounded metric series (60.3, 168.3, 219.1 mm and so on). The exact conversion is given here so the value can be rounded to whichever convention your drawing uses.
Copper tube. Copper to ASTM B88 uses its own nominal system in which the outside diameter is the nominal size plus 1/8 in — 5/8 in copper is 0.750 in outside. It is not NPS, and a copper “DN 15” is 15.88 mm outside against steel’s 21.34 mm.
Ductile and cast iron. AWWA C151 ductile iron has its own outside diameters, consistently larger than steel at the same DN — DN 80 is 100.58 mm against steel’s 88.90 mm. Iron pipe fittings and steel pipe fittings of the same DN do not interchange.
EN / DIN pipe. European pipe to EN 10220 and EN 10255 is specified by DN and outside diameter directly and has no NPS designation. Most sizes share the ASME outside diameters, but confirm against the standard rather than assuming.
Plastics. PVC to ASTM D1785 deliberately uses the steel outside diameters, which is why PVC Schedule 40 accepts steel-dimensioned fittings. Metric PE to EN 12201 does not — it is sized by true outside diameter.
- The NPS ↔ DN pairing is read from the same ASME B36.10 steel-pipe data the pipe-schedules page and the solver use, so it cannot drift from them; the DN designations follow ISO 6708 and EN 10220 practice.
- Outside diameters are the standard's inch values × 25.4 mm shown exactly (60.325 mm for NPS 2), not the rounded metric series (60.3) that some catalogues print; both describe the same pipe.
- NPS and DN are labels, not measurements: below NPS 14 the OD is larger than the nominal inch figure, from NPS 14 upward OD equals NPS in inches. The page marks where that change happens.
- Deliberately steel-only. Copper tube (ASTM B88) and ductile iron (AWWA C151) also use nominal sizes, but on different OD systems — mapping them onto this table would be wrong, so they are described in prose rather than tabulated.
Frequently Asked Questions
What is the difference between NPS and DN?
Is DN 50 the same as 2 inch pipe?
Why doesn't the nominal size match the pipe's actual diameter?
Does NPS ever equal the outside diameter?
Does the outside diameter change with schedule?
Do copper tube and ductile iron use the same DN sizes?
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