NPS 1/4 steel outer diameter: 0.540 in / 13.716 mm
- OD (inches)
- 0.540 in
- OD (millimetres)
- 13.716 mm
- DN
- 8
- Wall range
- 0.088–0.119 in
- Bore range
- 0.302–0.364 in
Welded and seamless wrought steel pipe. OD is fixed per size; the wall (schedule) is what you choose. For NPS 1/4, ASME B36.10M fixes the outside at 0.540 in / 13.716 mm; the 4 published wall series below are what change the bore.
| Wall series | Wall (in) | Wall (mm) | ID (in) | ID (mm) | Bore is | ≈ kg/m |
|---|---|---|---|---|---|---|
| Sch 40 | 0.088 | 2.235 | 0.364 | 9.246 | exact | 0.63 |
| Sch 80 | 0.119 | 3.023 | 0.302 | 7.671 | exact | 0.80 |
| STD | 0.088 | 2.235 | 0.364 | 9.246 | exact | 0.63 |
| XS | 0.119 | 3.023 | 0.302 | 7.671 | exact | 0.80 |
Same body, different material. steel 1/4 Sch 40 = STD, and stainless 1/4 Sch 40S are the same 0.088 in wall on the same 0.540 in / 13.716 mm body; steel 1/4 Sch 80 = XS, and stainless 1/4 Sch 80S are the same 0.119 in wall on the same 0.540 in / 13.716 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 | 0.540 | 13.716 | 8 | Sch 40 0.088 · Sch 80 0.119 · STD 0.088 · XS 0.119 |
| ASTM F876/F877 (CTS) · PEX tubing | 0.375 | 9.525 | 8 | no wall published yet (1 awaiting a second source) |
| ASME B36.19M · stainless steel pipe | 0.540 | 13.716 | 8 | Sch 10S 0.065 · Sch 40S 0.088 · Sch 80S 0.119 |
steel vs PEX — 1/4 (ASTM F876/F877 (CTS)) · steel vs stainless — 1/4 (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
13.716± a fraction of a millimetre points at NPS 1/4. 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 1/4 carries.
Questions about NPS 1/4
What is the outer diameter of NPS 1/4 pipe?
0.540 in / 13.716 mm — but that is only true for ASME B36.10M. The same nominal label in ASTM F876/F877 (CTS) (0.375 in / 9.525 mm) is a different pipe. Calipers measure this number; the name does not.
What is the inside diameter of NPS 1/4 Sch 40?
0.364 in / 9.246 mm — the outer diameter minus twice the wall (0.540 − 2 × 0.088). 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 1/4 the same as DN 8?
DN 8 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 1/4 measures 0.540 in / 13.716 mm outside. See the NPS ↔ DN table for the whole series.
Sources for these numbers
- Merit Brass
- Engineering ToolBox
- Wikipedia Nominal Pipe Size
- Enerpac NPS chart (PDF)
- 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