1/8 inch pipe size
10.3 mm outside diameter
NPS 1/8 · DN6 · 0.406 in OD · 6.8 mm bore in schedule 40
What this pipe is for
Instrument lines, gauge tappings and pneumatic control. Too small for anything carrying a useful flow.
The name is not the size
NPS 1/8 pipe is 0.406 inches across the outside, not 0.125. The name is 224% under the actual outside diameter, and it does not match the bore either — the schedule 40 inside diameter is 0.269 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 | 1/8 |
| DN (metric designation) | 6 |
| Outside diameter | 10.3 mm · 0.406 in |
| Wall, schedule 40 | 1.73 mm |
| Bore, schedule 40 | 6.8 mm · 0.269 in |
| Name against reality | OD is 3.24× 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 |
|---|---|---|---|
| 1/8 | 6 | 10.3 mm | 6.8 mm |
| 1/4 | 8 | 13.7 mm | 9.2 mm |
| 3/8 | 10 | 17.1 mm | 12.5 mm |
| 1/2 | 15 | 21.3 mm | 15.8 mm |
Frequently asked questions
What is the outside diameter of 1/8 inch pipe?
10.3 mm, or 0.406 inches. Note that this is not 0.125 inches — NPS 1/8 is a label, not a measurement, and the pipe is 224% larger across than its name.
What is the DN equivalent of NPS 1/8?
DN 6. DN is the metric designation and is just as nominal — DN 6 pipe is also 10.3 mm across, not 6. The two systems label the same pipe.
What is the inside diameter of 1/8 inch schedule 40 pipe?
About 6.8 mm (0.269 in), from an outside diameter of 10.3 mm and a 1.73 mm wall. Inside diameter changes with the schedule; outside diameter does not, which is why fittings are sized on it.
Why is NPS 1/8 not 0.125 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.