NPS 1 outer diameter: 1.315 in / 33.401 mm
- OD (inches)
- 1.315 in
- OD (millimetres)
- 33.401 mm
- DN
- 25
- Wall range
- 0.109–0.358 in
- Bore range
- 0.599–1.097 in
Welded and seamless wrought steel pipe. OD is fixed per size; the wall (schedule) is what you choose. For NPS 1, ASME B36.10M fixes the outside at 1.315 in / 33.401 mm; the 7 published wall series below are what change the bore.
| Wall series | Wall (in) | Wall (mm) | ID (in) | ID (mm) | Bore is | ≈ kg/m |
|---|---|---|---|---|---|---|
| Sch 10 | 0.109 | 2.769 | 1.097 | 27.864 | exact | 2.09 |
| Sch 40 | 0.133 | 3.378 | 1.049 | 26.645 | exact | 2.50 |
| Sch 80 | 0.179 | 4.547 | 0.957 | 24.308 | exact | 3.24 |
| Sch 160 | 0.250 | 6.350 | 0.815 | 20.701 | exact | 4.24 |
| STD | 0.133 | 3.378 | 1.049 | 26.645 | exact | 2.50 |
| XS | 0.179 | 4.547 | 0.957 | 24.308 | exact | 3.24 |
| XXS | 0.358 | 9.093 | 0.599 | 15.215 | exact | 5.45 |
Same body, different material. steel 1 Sch 40 = STD, and PVC 1 Sch 40, and stainless 1 Sch 40S are the same 0.133 in wall on the same 1.315 in / 33.401 mm body; steel 1 Sch 80 = XS, and PVC 1 Sch 80, and stainless 1 Sch 80S are the same 0.179 in wall on the same 1.315 in / 33.401 mm body. A caliper cannot separate them — what the pipe is made of does.
The same nominal label in other standards
| Standard | OD (in) | OD (mm) | DN | Published wall series |
|---|---|---|---|---|
| ASME B36.10M · welded and seamless steel pipe | 1.315 | 33.401 | 25 | Sch 10 0.109 · Sch 40 0.133 · Sch 80 0.179 · Sch 160 0.250 · STD 0.133 · XS 0.179 · XXS 0.358 |
| ASTM D2241 · PVC pressure pipe | 1.315 | 33.401 | 25 | Sch 40 0.133 · Sch 80 0.179 |
| ASTM F876/F877 (CTS) · PEX tubing (CTS) | 1.125 | 28.575 | 25 | SDR 9 0.125 |
| ASME B36.19M · stainless steel pipe | 1.315 | 33.401 | 25 | Sch 5S 0.065 · Sch 10S 0.109 · Sch 40S 0.133 · Sch 80S 0.179 |
steel vs PVC — 1 (ASTM D2241) · steel vs PEX — 1 (ASTM F876/F877 (CTS)) · steel vs stainless — 1 (ASME B36.19M)
Measuring this size
- Measure across the pipe, not the fitting: that is the OD, and it is the number a clamp, a hole saw or an insulation sleeve has to match.
- Measured
33.401± a fraction of a millimetre points at NPS 1. Check which standards a measurement fits — the same OD can belong to more than one. - The bore decides flow; the wall decides pressure rating and how deep a thread cuts. Neither number is in the name NPS 1 carries.
Questions about NPS 1
What is the outer diameter of NPS 1 pipe?
1.315 in / 33.401 mm — but that is only true for ASME B36.10M. The same nominal label in ASTM F876/F877 (CTS) (1.125 in / 28.575 mm) is a different pipe. Calipers measure this number; the name does not.
What is the inside diameter of NPS 1 Sch 40?
1.049 in / 26.645 mm — the outer diameter minus twice the wall (1.315 − 2 × 0.133). This standard publishes nominal wall, so the bore is exact within manufacturing tolerance. Other wall series for this size are in the table above.
Is NPS 1 the same as DN 25?
DN 25 is the millimetre-family name for this size, so yes for the size — no for the number. DN is not the outer diameter and not the bore; NPS 1 measures 1.315 in / 33.401 mm outside. See the NPS ↔ DN table for the whole series.
Sources for these numbers
- Enerpac NPS chart (PDF)
- SS Alloy steel pipe schedule
- Merit Brass
- Engineering ToolBox
- The Takeoff pipe schedule chart
- checked 2026-09-25
Each wall value in this page's tables was confirmed by two independent publications; the list above is those pairs combined, so a source appears once even when it supports several rows. The outer diameter is published per size, not per wall. Data version 48d86b977106.
No edition year is claimed. These values match ASME B36.10M as published by the sources above, but the specific printing was not verified, so the standard is named without a year.
Next
- Full ASME B36.10M chart (17 sizes)
- OD / ID / wall calculator — measure two, get the third
- About ASME B36.10M