NPS to DN Conversion Chart
Every nominal pipe size from NPS 1/8 to NPS 36 with its metric DN designator and its true outside diameter — because neither designator is a measurement of anything.
- Sizes listed
- 34
- Range
- NPS 1/8 – NPS 36
- Metric range
- DN 6 – DN 900
- Standard
- ASME B36.10M
NPS and DN are both designators: labels for a size, not measurements of it. Neither number is the outside diameter and neither is the bore. The only dimension that is real is the outside diameter in the third column, and that is what a tape measure will find on site.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Nominal pipe size | Metric designator | Outside diameter | OD (mm) | Full size data |
|---|---|---|---|---|
| NPS 1/8 | DN 6 | 0.40510.29 mm | 10.3 | dimensions → |
| NPS 1/4 | DN 8 | 0.54013.72 mm | 13.7 | dimensions → |
| NPS 3/8 | DN 10 | 0.67517.14 mm | 17.1 | dimensions → |
| NPS 1/2 | DN 15 | 0.84021.34 mm | 21.3 | dimensions → |
| NPS 3/4 | DN 20 | 1.05026.67 mm | 26.7 | dimensions → |
| NPS 1 | DN 25 | 1.31533.40 mm | 33.4 | dimensions → |
| NPS 1 1/4 | DN 32 | 1.66042.16 mm | 42.2 | dimensions → |
| NPS 1 1/2 | DN 40 | 1.90048.26 mm | 48.3 | dimensions → |
| NPS 2 | DN 50 | 2.37560.32 mm | 60.3 | dimensions → |
| NPS 2 1/2 | DN 65 | 2.87573.02 mm | 73.0 | dimensions → |
| NPS 3 | DN 80 | 3.50088.90 mm | 88.9 | dimensions → |
| NPS 3 1/2 | DN 90 | 4.000101.60 mm | 101.6 | dimensions → |
| NPS 4 | DN 100 | 4.500114.30 mm | 114.3 | dimensions → |
| NPS 4 1/2 | DN 115 | 5.000127.00 mm | 127.0 | dimensions → |
| NPS 5 | DN 125 | 5.563141.30 mm | 141.3 | dimensions → |
| NPS 6 | DN 150 | 6.625168.27 mm | 168.3 | dimensions → |
| NPS 7 | DN 175 | 7.625193.67 mm | 193.7 | dimensions → |
| NPS 8 | DN 200 | 8.625219.07 mm | 219.1 | dimensions → |
| NPS 9 | DN 225 | 9.625244.47 mm | 244.5 | dimensions → |
| NPS 10 | DN 250 | 10.750273.05 mm | 273.1 | dimensions → |
| NPS 11 | DN 275 | 11.750298.45 mm | 298.4 | dimensions → |
| NPS 12 | DN 300 | 12.750323.85 mm | 323.8 | dimensions → |
| NPS 14 | DN 350 | 14.000355.60 mm | 355.6 | dimensions → |
| NPS 16 | DN 400 | 16.000406.40 mm | 406.4 | dimensions → |
| NPS 18 | DN 450 | 18.000457.20 mm | 457.2 | dimensions → |
| NPS 20 | DN 500 | 20.000508.00 mm | 508.0 | dimensions → |
| NPS 22 | DN 550 | 22.000558.80 mm | 558.8 | dimensions → |
| NPS 24 | DN 600 | 24.000609.60 mm | 609.6 | dimensions → |
| NPS 26 | DN 650 | 26.000660.40 mm | 660.4 | dimensions → |
| NPS 28 | DN 700 | 28.000711.20 mm | 711.2 | dimensions → |
| NPS 30 | DN 750 | 30.000762.00 mm | 762.0 | dimensions → |
| NPS 32 | DN 800 | 32.000812.80 mm | 812.8 | dimensions → |
| NPS 34 | DN 850 | 34.000863.60 mm | 863.6 | dimensions → |
| NPS 36 | DN 900 | 36.000914.40 mm | 914.4 | dimensions → |
The conversion is a lookup, not a calculation
DN is not NPS × 25.4. DN 50 pipe is NPS 2, and NPS 2 is 2.375 in — which is 60.3 mm, not 50 mm. The DN series was chosen to be a convenient round-number set that maps one-to-one onto the existing inch sizes, so the only reliable way to convert is to read the table.
Where the numbers start telling the truth
From NPS 14 upward, NPS equals the outside diameter in inches exactly: NPS 14 is 14.000 in, NPS 36 is 36.000 in. Below NPS 14 the designator is inherited from nineteenth-century wrought iron pipe, where the number described the approximate bore of the thick-walled pipe of the day. Walls got thinner, bores got bigger, and the label stayed.
Common questions
Is DN the same as the pipe diameter in millimetres?
No. DN is a dimensionless designator. DN 50 pipe has an outside diameter of 60.3 mm, not 50 mm. Treating DN as a measurement is one of the most common sizing mistakes.
How do I convert NPS to DN?
By table lookup only. There is no formula: the two series were aligned by agreement, not by arithmetic. NPS 2 is DN 50, NPS 4 is DN 100, NPS 10 is DN 250.
What does NPS actually measure?
Nothing, directly. Below NPS 14 it is a historic label that matches neither the OD nor the bore. From NPS 14 up it happens to equal the outside diameter in inches.