NPS 1/8 Pipe Dimensions
Outside diameter 0.405 in (10.3 mm) — fixed across every schedule. 8 wall thicknesses from ASME B36.10, with bore, weight, flow and pressure capacity for each, and the flanges and fittings that go with the size.
- Nominal size
- NPS 1/8
- Metric designator
- DN 6
- Outside diameter
- 0.405 in (10.3 mm)
- Schedules listed
- 8
- Standard weight wall
- 0.068 in (1.7 mm)
- Extra strong wall
- 0.095 in (2.4 mm)
What NPS 1/8 is
NPS 1/8 pipe measures 0.405 in (10.3 mm) across the outside, which is 0.280 in more than the 1/8 in its name suggests. Below NPS 14 the designation is a label inherited from iron pipe sizing and matches neither the outside diameter nor the bore of any schedule exactly. The metric designator is DN 6, which is likewise a label and not a measurement in millimetres. Whatever the schedule, the outside diameter stays at 0.405 in (10.3 mm): a heavier wall grows inward and takes the difference out of the bore.
This is the smallest size ASME B36.10M publishes. It turns up on gauge and instrument connections, lubrication lines and sample points rather than on process lines, and it is normally threaded: there is too little wall to butt-weld comfortably.
Every schedule in NPS 1/8
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Schedule | Wall thickness | Inside diameter | Weight, empty | Weight, water filled | Flow area (in²) | Water (US gal/ft) |
|---|---|---|---|---|---|---|
| Sch 10 | 0.0491.24 mm | 0.3077.80 mm | 0.190.28 kg/m | 0.220.33 kg/m | 0.07 | 0.004 |
| Sch 30 | 0.0571.45 mm | 0.2917.39 mm | 0.210.32 kg/m | 0.240.36 kg/m | 0.07 | 0.003 |
| Sch 40 | 0.0681.73 mm | 0.2696.83 mm | 0.240.36 kg/m | 0.270.40 kg/m | 0.06 | 0.003 |
| STD | 0.0681.73 mm | 0.2696.83 mm | 0.240.36 kg/m | 0.270.40 kg/m | 0.06 | 0.003 |
| Sch 80 | 0.0952.41 mm | 0.2155.46 mm | 0.310.47 kg/m | 0.330.49 kg/m | 0.04 | 0.002 |
| XS | 0.0952.41 mm | 0.2155.46 mm | 0.310.47 kg/m | 0.330.49 kg/m | 0.04 | 0.002 |
| Sch 160 | 0.1243.15 mm | 0.1573.99 mm | 0.370.55 kg/m | 0.380.57 kg/m | 0.02 | 0.001 |
| XXS | 0.1904.83 mm | 0.0250.64 mm | 0.440.65 kg/m | 0.440.65 kg/m | 0.00 | 0.000 |
Inside diameter is OD − 2t. Weight is calculated for carbon steel as w = 10.69 × t × (OD − t) lb/ft and is the plain-end weight, excluding coatings, linings and fittings. Water-filled weight adds fresh water at 8.3454 lb per US gallon.
Reading the table
B36.10 lists 8 designations in NPS 1/8, which between them give 6 different wall thicknesses. The lightest is Schedule 10 at 0.049 in (1.2 mm) and the heaviest is Double Extra Strong (XXS) at 0.190 in (4.8 mm). Going from one to the other takes the bore from 0.307 in (7.8 mm) down to 0.025 in (0.6 mm), which cuts the flow area by 99% and raises the weight from 0.19 to 0.44 lb/ft, a factor of 2.3.
Standard weight equals Schedule 40 in this size. Extra strong equals Schedule 80. Where two designations share a wall they are the same pipe under two names, and either may appear on a mill certificate. The match is specific to the size, so it is the wall thickness in inches that should go on a requisition, not the designation alone.
For handling and support loads, a 20 ft length of standard weight NPS 1/8 pipe weighs about 5 lb (2 kg) empty and 5 lb (2 kg) full of water. The water-filled figure is the one that governs pipe support spacing and the load during a hydrostatic test, including on lines that will carry gas or vapour in service.
Flow and pressure capacity by schedule
Two questions decide a schedule: how much the bore will carry, and how much pressure the wall will hold. The flow columns give the water flow through each bore at three velocities that bracket ordinary liquid service, from a pump suction line at the low end to a pump discharge line at the high end. The pressure columns are the ASME B31.3 pressure design equation solved for pressure, first on the nominal wall and then on the wall after the 12.5% mill under-tolerance that seamless pipe is allowed.
| Schedule | Flow at 3 ft/s (US gpm) | Flow at 6 ft/s (US gpm) | Flow at 10 ft/s (US gpm) | Pressure, nominal wall | Pressure, wall less 12.5% |
|---|---|---|---|---|---|
| Sch 10 | 0.7 | 1.4 | 2.3 | 5,358 psig | 4,626 psig |
| Sch 30 | 0.6 | 1.2 | 2.1 | 6,344 psig | 5,464 psig |
| Sch 40 | 0.5 | 1.1 | 1.8 | — | — |
| STD | 0.5 | 1.1 | 1.8 | — | — |
| Sch 80 | 0.3 | 0.7 | 1.1 | — | — |
| XS | 0.3 | 0.7 | 1.1 | — | — |
| Sch 160 | 0.2 | 0.4 | 0.6 | — | — |
| XXS | 0.0 | 0.0 | 0.0 | — | — |
Flow is velocity × flow area. Pressure is P = 2·S·t / (D − 2·Y·t) with S = 20,000 psi, E = W = 1.0 and Y = 0.4: seamless carbon steel such as A106 Gr B at moderate temperature, with no corrosion allowance and no threading allowance. It is an illustration of the arithmetic, not a design rating. A dash marks a wall of D/6 or more, where that equation does not apply.
The method behind both halves of that table is set out in the pipe sizing guide and the wall thickness guide, each with a worked example.
Flanges for NPS 1/8 pipe
ASME B16.5 publishes no flange in NPS 1/8 and ASME B16.9 no buttweld fitting. That is the practical meaning of an obsolete size: pipe of this diameter exists in old plant, but nothing standard is made to connect to it. New work uses the next size up or down.
Guides and comparisons for this page
NPS 1/8 in each schedule chart
Common questions
What is the outside diameter of NPS 1/8 pipe?
NPS 1/8 pipe has an outside diameter of 0.405 in (10.3 mm). That outside diameter is fixed: it is identical in every schedule, and only the wall thickness — and therefore the bore — changes from one schedule to the next.
What is the inside diameter of NPS 1/8 schedule 40 pipe?
0.269 in (6.8 mm). The 0.068 in (1.7 mm) wall comes off the 0.405 in (10.3 mm) outside diameter twice, once on each side, which leaves that bore and a flow area of 0.06 in².
How much does NPS 1/8 schedule 40 pipe weigh?
0.24 lb/ft (0.36 kg/m) as plain-end carbon steel, on a wall thickness of 0.068 in (1.7 mm). It holds 0.003 US gallons of water per foot.
What is the metric equivalent of NPS 1/8?
DN 6. DN is a dimensionless designator, not a measurement — DN 6 pipe does not measure 6 mm anywhere. Its actual outside diameter is 10.3 mm.