NPS 12 steel outer diameter: 12.750 in / 323.85 mm
- OD (inches)
- 12.750 in
- OD (millimetres)
- 323.85 mm
- DN
- 300
- Wall range
- 0.180–1.312 in
- Bore range
- 10.126–12.390 in
Welded and seamless wrought steel pipe. OD is fixed per size; the wall (schedule) is what you choose. For NPS 12, ASME B36.10M fixes the outside at 12.750 in / 323.85 mm; the 13 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 | 0.180 | 4.572 | 12.390 | 314.706 | exact | 36.00 |
| Sch 20 | 0.250 | 6.350 | 12.250 | 311.150 | exact | 49.72 |
| Sch 30 | 0.330 | 8.382 | 12.090 | 307.086 | exact | 65.21 |
| Sch 40 | 0.406 | 10.312 | 11.938 | 303.225 | exact | 79.74 |
| Sch 60 | 0.562 | 14.275 | 11.626 | 295.300 | exact | 108.98 |
| Sch 80 | 0.688 | 17.475 | 11.374 | 288.900 | exact | 132.04 |
| Sch 100 | 0.844 | 21.438 | 11.062 | 280.975 | exact | 159.88 |
| Sch 120 | 1.000 | 25.400 | 10.750 | 273.050 | exact | 186.95 |
| Sch 140 | 1.125 | 28.575 | 10.500 | 266.700 | exact | 208.08 |
| Sch 160 | 1.312 | 33.325 | 10.126 | 257.200 | exact | 238.77 |
| STD | 0.375 | 9.525 | 12.000 | 304.800 | exact | 73.84 |
| XS | 0.500 | 12.700 | 11.750 | 298.450 | exact | 97.45 |
| XXS | 1.000 | 25.400 | 10.750 | 273.050 | exact | 186.95 |
Same body, different material. steel 12 Sch 10, and stainless 12 Sch 10S are the same 0.180 in wall on the same 12.750 in / 323.85 mm body; steel 12 STD, and stainless 12 Sch 40S are the same 0.375 in wall on the same 12.750 in / 323.85 mm body. A caliper cannot separate them — what the pipe is made of does.
One wall, 2 names. For this size NPS 12 Sch 120 = NPS 12 XXS are the same physical wall (25.400 mm). That equality is size-specific: it does not hold for the whole standard.
The same nominal label in other standards
| Standard | OD (in) | OD (mm) | DN | Published wall series |
|---|---|---|---|---|
| ASME B36.10M · welded and seamless steel pipe | 12.750 | 323.850 | 300 | Sch 10 0.180 · Sch 20 0.250 · Sch 30 0.330 · Sch 40 0.406 · Sch 60 0.562 · Sch 80 0.688 · Sch 100 0.844 · Sch 120 1.000 · Sch 140 1.125 · Sch 160 1.312 · STD 0.375 · XS 0.500 · XXS 1.000 |
| ASME B36.19M · stainless steel pipe | 12.750 | 323.850 | 300 | Sch 5S 0.156 · Sch 10S 0.180 · Sch 40S 0.375 |
steel vs stainless — 12 (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
323.85± a fraction of a millimetre points at NPS 12. 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 12 carries.
Questions about NPS 12
What is the outer diameter of NPS 12 pipe?
12.750 in / 323.85 mm — but that is only true for ASME B36.10M. The same nominal label in another standard is a different pipe. Calipers measure this number; the name does not.
What is the inside diameter of NPS 12 Sch 40?
11.938 in / 303.225 mm — the outer diameter minus twice the wall (12.750 − 2 × 0.406). 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 12 the same as DN 300?
DN 300 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 12 measures 12.750 in / 323.85 mm outside. See the NPS ↔ DN table for the whole series.
Do NPS 12 Sch 120 and NPS 12 XXS mean different pipes?
No. At this size they are 2 names for one wall: 1.000 in / 25.400 mm on the same 12.750 in / 323.85 mm body. Choose the name your supplier or drawing uses — and do not carry the equality to other sizes; it holds only where the published walls are actually equal.
Sources for these numbers
- Enerpac NPS chart (PDF)
- SS Alloy steel pipe schedule
- The Takeoff pipe schedule chart
- Octal Steel - ANSI B36.10 & B36.19 dimension chart (PDF)
- Engineering ToolBox
- Wikipedia Nominal Pipe Size
- 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