NPS vs DN Explained
Two designation systems, neither of which measures the pipe. Why DN is not NPS × 25.4, and where treating either one as a dimension goes wrong.
- NPS
- Nominal Pipe Size — dimensionless
- DN
- Diamètre Nominal — dimensionless
- Conversion
- Table lookup, never arithmetic
- NPS = OD from
- NPS 14 upward
Short answer. NPS and DN are both designators — labels for a size, not measurements of anything. DN 50 pipe is not 50 mm in any dimension: it is NPS 2, whose outside diameter is 2.375 in or 60.3 mm. The two series were aligned by agreement rather than by arithmetic, so converting between them is always a table lookup and never a calculation.
Neither number measures the pipe
Below NPS 14, the NPS number matches neither the outside diameter nor the bore. It is inherited from nineteenth-century wrought iron pipe, where the number described the approximate inside diameter of the thick-walled pipe of the day. Walls got thinner, bores got larger, and the labels stayed where they were.
From NPS 14 upward the designator finally tells the truth: NPS equals the outside diameter in inches exactly. NPS 14 pipe is 14.000 in outside, NPS 24 is 24.000 in. The last column of the table below marks where that changeover happens.
DN ≠ NPS × 25.4 · DN 50 = NPS 2 = 60.3 mm OD
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Nominal pipe size | Metric designator | True outside diameter | DN as a number | OD minus DN | NPS = OD? |
|---|---|---|---|---|---|
| NPS 1/8 | DN 6 | 0.40510.29 mm | 6 mm | 4.3 mm | No |
| NPS 1/4 | DN 8 | 0.54013.72 mm | 8 mm | 5.7 mm | No |
| NPS 3/8 | DN 10 | 0.67517.14 mm | 10 mm | 7.1 mm | No |
| NPS 1/2 | DN 15 | 0.84021.34 mm | 15 mm | 6.3 mm | No |
| NPS 3/4 | DN 20 | 1.05026.67 mm | 20 mm | 6.7 mm | No |
| NPS 1 | DN 25 | 1.31533.40 mm | 25 mm | 8.4 mm | No |
| NPS 1 1/4 | DN 32 | 1.66042.16 mm | 32 mm | 10.2 mm | No |
| NPS 1 1/2 | DN 40 | 1.90048.26 mm | 40 mm | 8.3 mm | No |
| NPS 2 | DN 50 | 2.37560.32 mm | 50 mm | 10.3 mm | No |
| NPS 2 1/2 | DN 65 | 2.87573.02 mm | 65 mm | 8.0 mm | No |
| NPS 3 | DN 80 | 3.50088.90 mm | 80 mm | 8.9 mm | No |
| NPS 3 1/2 | DN 90 | 4.000101.60 mm | 90 mm | 11.6 mm | No |
| NPS 4 | DN 100 | 4.500114.30 mm | 100 mm | 14.3 mm | No |
| NPS 4 1/2 | DN 115 | 5.000127.00 mm | 115 mm | 12.0 mm | No |
| NPS 5 | DN 125 | 5.563141.30 mm | 125 mm | 16.3 mm | No |
| NPS 6 | DN 150 | 6.625168.27 mm | 150 mm | 18.3 mm | No |
| NPS 7 | DN 175 | 7.625193.67 mm | 175 mm | 18.7 mm | No |
| NPS 8 | DN 200 | 8.625219.07 mm | 200 mm | 19.1 mm | No |
| NPS 9 | DN 225 | 9.625244.47 mm | 225 mm | 19.5 mm | No |
| NPS 10 | DN 250 | 10.750273.05 mm | 250 mm | 23.1 mm | No |
| NPS 11 | DN 275 | 11.750298.45 mm | 275 mm | 23.4 mm | No |
| NPS 12 | DN 300 | 12.750323.85 mm | 300 mm | 23.8 mm | No |
| NPS 14 | DN 350 | 14.000355.60 mm | 350 mm | 5.6 mm | Yes |
| NPS 16 | DN 400 | 16.000406.40 mm | 400 mm | 6.4 mm | Yes |
| NPS 18 | DN 450 | 18.000457.20 mm | 450 mm | 7.2 mm | Yes |
| NPS 20 | DN 500 | 20.000508.00 mm | 500 mm | 8.0 mm | Yes |
| NPS 22 | DN 550 | 22.000558.80 mm | 550 mm | 8.8 mm | Yes |
| NPS 24 | DN 600 | 24.000609.60 mm | 600 mm | 9.6 mm | Yes |
The “OD minus DN” column shows how far the metric designator sits from the real dimension — from a few millimetres in small bore to tens of millimetres in large.
Why DN is not NPS × 25.4
Because DN was chosen to be a tidy round-number series that maps one-to-one onto the existing inch sizes, not to be a conversion of them. NPS 2 is 2.375 in, which is 60.3 mm — and its designator is DN 50. The 50 is a label picked for convenience.
The series is also not evenly spaced. It runs 15, 20, 25, 32, 40, 50, 65, 80, 100 — matching the irregular inch progression beneath it. Any attempt to compute one from the other breaks immediately.
Other terms you will meet
| Term | Means | Note |
|---|---|---|
| NPS | Nominal Pipe Size | The North American designator. Dimensionless. |
| DN | Diamètre Nominal / Diameter Nominal | The ISO metric designator. Also dimensionless. |
| NB | Nominal Bore | Used interchangeably with NPS in British and Commonwealth practice. Same numbers as NPS. |
| OD | Outside diameter | A real measurement — the only one on this list. |
| ID | Inside diameter / bore | A real measurement, but it changes with schedule. |
| NPT | National Pipe Taper thread | A thread standard, not a size designator. Frequently confused with NPS. |
NPS, DN and NB are labels. OD and ID are dimensions.
Where this actually bites
- Ordering from a metric supplier. Asking for 50 mm pipe may get DN 50 (NPS 2, 60.3 mm OD) or genuine 50 mm OD tube. They are different products and do not share fittings.
- Pipe versus tube. Tube is specified by actual outside diameter and wall. Pipe is specified by NPS and schedule. A 2 in tube and NPS 2 pipe are not the same size.
- Drawing take-offs. Using DN as a millimetre dimension in a clash check or an insulation calculation understates every diameter on the drawing.
- Hydraulic calculations. Neither designator is the bore. The bore depends on schedule and must be read from the dimension table.
See also: the full NPS to DN conversion chart · what a schedule is · all pipe sizes · pipe sizing.
Common questions
Is DN 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 errors.
How do I convert NPS to DN?
By table lookup. There is no formula — the two series were aligned by agreement. NPS 2 is DN 50, NPS 4 is DN 100, NPS 10 is DN 250.
Is NB the same as NPS?
Effectively yes. Nominal Bore is the British and Commonwealth term for the same designator and uses the same numbers.
When does NPS equal the actual outside diameter?
From NPS 14 upward. NPS 14 pipe is exactly 14.000 in outside. Below that the designator matches neither the outside diameter nor the bore.
What is the difference between pipe and tube sizing?
Pipe is specified by NPS and schedule, where NPS is a label. Tube is specified by its actual outside diameter and wall thickness. A 2 in tube is 2.000 in across; NPS 2 pipe is 2.375 in.