Pipe sizing is confusing because the nominal name rarely matches any real dimension. This reference resolves an NPS or DN size into its true outside diameter, then gives wall thickness and inside diameter for the common schedules.
Why nominal size does not equal real size
The NPS system evolved from the era of threaded iron pipe. When the pipe wall was thickened for higher pressures, the outside diameter was kept constant so that fittings and threads could remain interchangeable. The inside diameter shrank, but the nominal name did not change. By the time standards were formalised, the “nominal” label was already a historical artefact with no direct geometric meaning for sizes up to NPS 12.
From NPS 14 upward the nominal size does equal the outside diameter in inches — the system was rationalised for larger industrial pipe where threads are not used.
How it works
For each nominal size the outside diameter (OD) is fixed by standard so threads and fittings interchange. The schedule sets the wall thickness. The bore follows directly:
inside diameter = outside diameter - (2 × wall thickness)For example, DN50 / NPS 2 has a fixed OD of 60.3 mm. At schedule 40 the wall is 3.91 mm, so the bore is 60.3 - 2 × 3.91 = 52.5 mm. At schedule 80 the wall thickens to 5.54 mm and the bore shrinks to about 49.2 mm — the same nominal size, the same OD, but a smaller flow area.
Selected reference values
| NPS | DN | OD (mm) | Sch 40 wall (mm) | Sch 40 ID (mm) | Sch 80 wall (mm) | Sch 80 ID (mm) |
|---|---|---|---|---|---|---|
| 1/2 | DN15 | 21.3 | 2.77 | 15.8 | 3.73 | 13.8 |
| 1 | DN25 | 33.4 | 3.38 | 26.6 | 4.55 | 24.3 |
| 2 | DN50 | 60.3 | 3.91 | 52.5 | 5.54 | 49.2 |
| 4 | DN100 | 114.3 | 6.02 | 102.3 | 8.56 | 97.2 |
| 6 | DN150 | 168.3 | 7.11 | 154.1 | 10.97 | 146.4 |
Choosing the right schedule
- Schedule 40 (standard weight) is the common default for domestic and commercial water, gas, and low-pressure industrial services.
- Schedule 80 (extra heavy) is used where higher pressure ratings or additional wall strength are needed — chemical lines, high-pressure steam, or where physical protection matters.
- Schedule 160 and XXS (double extra strong) appear in high-pressure hydraulics and specialised industrial applications.
The schedule choice is a pressure and mechanical rating decision, not a flow decision. For a given flow rate, a higher schedule reduces the bore and increases velocity — check the resulting pressure drop before specifying a heavier schedule on a long run.