12 inch pipe size
323.8 mm outside diameter
NPS 12 · DN300 · 12.748 in OD · 303.2 mm bore in schedule 40
What this pipe is for
The last size where the NPS number is still smaller than the outside diameter — 12 inch pipe is 12.75 inches across.
The name is not the size
NPS 12 pipe is 12.748 inches across the outside, not 12. The name is 6% under the actual outside diameter, and it does not match the bore either — the schedule 40 inside diameter is 11.936 inches.
The label is inherited from wrought-iron pipe, whose much heavier walls gave roughly this bore at this outside diameter. When pipe walls got thinner the outside diameter had to stay put, because the threads and fittings were already standardised on it — so the name froze while the thing it described moved.
The numbers
| NPS | 12 |
| DN (metric designation) | 300 |
| Outside diameter | 323.8 mm · 12.748 in |
| Wall, schedule 40 | 10.31 mm |
| Bore, schedule 40 | 303.2 mm · 11.936 in |
| Name against reality | OD is 1.06× the NPS number |
Outside diameter is the number that matters. It is fixed for a given NPS regardless of wall thickness, which is what lets a fitting made for schedule 10 thread onto schedule 80. The bore changes with the schedule; the outside never does.
Nearby sizes
| NPS | DN | OD | Sch 40 bore |
|---|---|---|---|
| 6 | 150 | 168.3 mm | 154.1 mm |
| 8 | 200 | 219.1 mm | 202.7 mm |
| 10 | 250 | 273 mm | 254.5 mm |
| 12 | 300 | 323.8 mm | 303.2 mm |
| 14 | 350 | 355.6 mm | 333.3 mm |
| 16 | 400 | 406.4 mm | 381.0 mm |
| 18 | 450 | 457.2 mm | 428.7 mm |
| 20 | 500 | 508 mm | 477.8 mm |
| 24 | 600 | 610 mm | 575.0 mm |
Frequently asked questions
What is the outside diameter of 12 inch pipe?
323.8 mm, or 12.748 inches. Note that this is not 12 inches — NPS 12 is a label, not a measurement, and the pipe is 6% larger across than its name.
What is the DN equivalent of NPS 12?
DN 300. DN is the metric designation and is just as nominal — DN 300 pipe is also 323.8 mm across, not 300. The two systems label the same pipe.
What is the inside diameter of 12 inch schedule 40 pipe?
About 303.2 mm (11.936 in), from an outside diameter of 323.8 mm and a 10.31 mm wall. Inside diameter changes with the schedule; outside diameter does not, which is why fittings are sized on it.
Why is NPS 12 not 12 inches?
Because the name is inherited. NPS came from wrought-iron pipe whose much thicker walls gave roughly this bore, and the label was kept when the pipe changed, because the threads and fittings could not change with it. Below NPS 14 the number matches neither the outside nor the inside diameter of modern pipe.