NPS 1/4 Pipe Dimensions

Nominal size

NPS 1/4

Metric designator

DN 8

Outside diameter

0.540 in (13.7 mm)

Schedules listed

8

Standard weight wall

0.088 in (2.2 mm)

Extra strong wall

0.119 in (3.0 mm)

What NPS 1/4 is

NPS 1/4 pipe measures 0.540 in (13.7 mm) across the outside, which is 0.290 in more than the 1/4 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 8, which is likewise a label and not a measurement in millimetres. Whatever the schedule, the outside diameter stays at 0.540 in (13.7 mm): a heavier wall grows inward and takes the difference out of the bore.

A size for instrument and gauge connections, lube oil and seal lines, and small air or sample tubing runs where pipe is preferred to tube. It is normally threaded or socket-welded.

Every schedule in NPS 1/4

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

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 1/4, which between them give 6 different wall thicknesses. The lightest is Schedule 10 at 0.065 in (1.7 mm) and the heaviest is Double Extra Strong (XXS) at 0.238 in (6.0 mm). Going from one to the other takes the bore from 0.410 in (10.4 mm) down to 0.064 in (1.6 mm), which cuts the flow area by 98% and raises the weight from 0.33 to 0.77 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 1/4 pipe weighs about 9 lb (4 kg) empty and 9 lb (4 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.

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 1/4 pipe

ASME B16.5 publishes no flange in NPS 1/4 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 1/4 in each schedule chart

Adjacent sizes