DN to NPS conversion
DN and NPS are two names for the same nominal size, one metric and one imperial. The mapping is exact and the outside diameter is a third, separate number — which is why a conversion table is necessary and why it is not enough.
DN is not a millimetre dimension. DN 100 pipe is 114.3 mm outside, not 100 mm. Anyone who reads "DN" as "outside diameter in mm" will size every fitting and every support on a line 14 % wrong. DN is dimensionless by definition — the abbreviation is Diamètre Nominal — and the number is a label inherited from a nineteenth-century cast-iron sizing convention where it happened to be close to the bore.
The conversion
Nominal inches from DN
NPS (in) ≈ DN ÷ 25.4 = DN × 0.0394
The result is a nominal label, not a dimension. Round it to the tabulated NPS value: DN 50 → 1.969 in → NPS 2.
Worked through with the default dimensions:
DN = 50 (dimensionless label)NPS = 50 ÷ 25.4 = 1.969 inNearest tabulated NPS = 2Actual outside diameter of NPS 2 / DN 50 = 2.375 in = 60.3 mmSchedule 40 wall = 3.91 mm → bore = 52.5 mm
- DN
- Metric nominal designation, dimensionless
- NPS
- Imperial nominal designation, written in inches but not a measurement
- OD
- Outside diameter — the only real dimension among these three, from ASME B36.10M
There is nothing to calculate once the pairing is known: DN and NPS are paired by definition in the standard, and the conversion above only exists so the pairing can be reconstructed when a drawing gives one system and a supplier quotes the other.
Complete table
| Metric designation | Imperial designation | Real outside dia. mm | in | DN expressed in inches in | Schedules |
|---|---|---|---|---|---|
| DN 6 | NPS 1/8 | 10.3 | 0.405 | 0.24 | 10 |
| DN 8 | NPS 1/4 | 13.7 | 0.540 | 0.31 | 10 |
| DN 10 | NPS 3/8 | 17.1 | 0.675 | 0.39 | 10 |
| DN 15 | NPS 1/2 | 21.3 | 0.840 | 0.59 | 12 |
| DN 20 | NPS 3/4 | 26.7 | 1.050 | 0.79 | 12 |
| DN 25 | NPS 1 | 33.4 | 1.315 | 0.98 | 12 |
| DN 32 | NPS 1-1/4 | 42.2 | 1.660 | 1.26 | 12 |
| DN 40 | NPS 1-1/2 | 48.3 | 1.900 | 1.57 | 12 |
| DN 50 | NPS 2 | 60.3 | 2.375 | 1.97 | 12 |
| DN 65 | NPS 2-1/2 | 73.0 | 2.875 | 2.56 | 13 |
| DN 80 | NPS 3 | 88.9 | 3.500 | 3.15 | 13 |
| DN 90 | NPS 3-1/2 | 101.6 | 4.000 | 3.54 | 11 |
| DN 100 | NPS 4 | 114.3 | 4.500 | 3.94 | 13 |
| DN 115 | NPS 4-1/2 | 127.0 | 5.000 | 4.53 | 7 |
| DN 125 | NPS 5 | 141.3 | 5.563 | 4.92 | 12 |
| DN 150 | NPS 6 | 168.3 | 6.625 | 5.91 | 12 |
| DN 200 | NPS 8 | 219.1 | 8.625 | 7.87 | 17 |
| DN 250 | NPS 10 | 273.1 | 10.750 | 9.84 | 16 |
| DN 300 | NPS 12 | 323.8 | 12.750 | 11.81 | 16 |
| DN 350 | NPS 14 | 355.6 | 14.000 | 13.78 | 16 |
| DN 400 | NPS 16 | 406.4 | 16.000 | 15.75 | 16 |
| DN 450 | NPS 18 | 457.2 | 18.000 | 17.72 | 16 |
| DN 500 | NPS 20 | 508.0 | 20.000 | 19.69 | 16 |
| DN 550 | NPS 22 | 558.8 | 22.000 | 21.65 | 15 |
| DN 600 | NPS 24 | 609.6 | 24.000 | 23.62 | 16 |
| DN 650 | NPS 26 | 660.4 | 26.000 | 25.59 | 5 |
| DN 700 | NPS 28 | 711.2 | 28.000 | 27.56 | 6 |
| DN 750 | NPS 30 | 762.0 | 30.000 | 29.53 | 8 |
| DN 800 | NPS 32 | 812.8 | 32.000 | 31.50 | 7 |
| DN 850 | NPS 34 | 863.6 | 34.000 | 33.46 | 7 |
| DN 900 | NPS 36 | 914.4 | 36.000 | 35.43 | 7 |
The two nominal columns are labels; only the outside diameter columns are dimensions. Where 'DN expressed in inches' and 'real outside dia.' differ, that difference is the whole point of this page.
Each size page lists every schedule and the bore each one gives: NPS 1/8 · NPS 1/4 · NPS 3/8 · NPS 1/2 · NPS 3/4 · NPS 1 … and the rest on the nominal sizes page .
Where the two systems diverge
- The small end. DN 6, 8 and 10 are NPS 1/8, 1/4 and 3/8. These designations exist in both systems but the metric sizes are effectively never used — small instrumentation tubing is specified in millimetres of actual outside diameter instead.
- NPS 4-1/2 and NPS 22. Live in NPS, mapped to DN 115 and DN 550, and uncommon enough that they are regularly missed when someone builds a size list by hand.
- Above NPS 36. The imperial series stops being tabulated in the common schedule charts while DN continues. A large-diameter line is therefore specified in millimetres with an actual outside diameter, and the DN designation becomes decorative.
Download the conversion table
DN / NPS / outside diameter
Reads the table above. The pipe dataset on the downloads page carries every size against every schedule with wall, bore and mass.
Frequently asked questions
How do I convert DN to NPS?
Multiply DN by 0.0394 (divide by 25.4) to get the nominal inches, then accept that the result is nominal rather than precise. DN 50 → 1.97 in → NPS 2. The mapping is exact by definition — DN was created as the metric counterpart of NPS, not measured independently — so a conversion table is a list of labels, not a calculation. What you cannot do is derive the outside diameter from DN: DN 50 pipe is 60.3 mm outside.
Is DN the same as the outside diameter in millimetres?
No, and this is the most common mistake in the whole subject. DN is dimensionless by definition — "Diamètre Nominal" — and for most sizes it is smaller than the outside diameter. DN 100 (NPS 4) pipe is 114.3 mm outside. The relationship happened to hold for the old cast-iron sizes that gave the system its numbers, and it has not held since. Always get the outside diameter from the schedule chart, never from the DN number.
Which sizes differ between the two systems?
Four cases are worth knowing. At the small end, DN 6, 8 and 10 map to NPS 1/8, 1/4 and 3/8 — designations the metric world effectively never uses. NPS 4-1/2 is DN 115 and NPS 22 is DN 550; both are real but uncommon in one system or the other. And above NPS 36 (DN 900) the imperial series effectively stops while metric sizes continue, so a large-diameter line is normally specified in millimetres outright.
Why does the conversion table show a DN in inches?
So you can see how loosely the two systems align. DN 15 is 0.59 in, which is "NPS 1/2" — the two numbers are not the same quantity expressed twice, they are two independent labels that happen to be paired. The inches column is there to make that obvious rather than to be used.
Pipe reference and tools
Once the size is settled, the schedule decides the wall — and the wall decides the bore.
- Pipe overview — schedules, sizes and how weight is computed
- All pipe schedules — Sch 5S to XXS
- All nominal sizes — NPS 1/8 to NPS 36
- DN to NPS conversion — metric to imperial designations
- Pipe weight formula — worked derivation
- Pipe weight calculator — any OD and wall
- All calculators — metal weight and pipe weight
- Download the data — CSV, generated from the same source
- Cross-reference tables — DN / NPS / material equivalents
- Methodology and sources — where every number comes from
DN values are the standard metric designations paired with each nominal pipe size. Outside diameters are the ASME B36.10M values in inches, converted at exactly 25.4 mm per inch. No outside diameter here is derived from a DN value.
- ASME B36.10M — Welded and Seamless Wrought Steel Pipe (dimensional tables).
- BIPM / NIST definitional constants: international inch = 25.4 mm, foot = 0.3048 m, pound = 0.45359237 kg.
Dataset version 2026.09.1 · dimensions last verified against source 2026-09-21. Found an error? Report it — corrections are issued with a version bump.