FittingKey

Fittings · NPT / NPTF

American National Pipe Taper, NPT (ASME B1.20.1) and NPTF dryseal (ASME B1.20.3)

3/8 NPT 3/8-18 NPT

3/8 in trade size. Tapered thread, sealed on the thread, tapered thread, 60 degree flank angle. Basic major diameter 17.15 mm / 0.675 in.

Identify a fitting you are holdingAll NPT / NPTF sizes
Measure across the crests
16.6 mm to 17.3 mm
Basic 17.15 mm / 0.675 in

The visual tell

What the end of the fitting looks like

Tapered thread, sealed on the thread, on a tapered threadNo seat at allTapered thread, 1 in 16Open end on the left
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it. Thread shown: 3/8-18 NPT.

No cone, no groove, no flat sealing face - just a thread that visibly narrows towards the end of the fitting.

Where it seals: The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread.

Dimensions

Thread size, pitch and diameters for 3/8 NPT.
Designation3/8 NPT
Thread3/8-18 NPT
Threads per inch / pitch18 TPI (1.411 mm pitch)
Basic major diameter (male), at the gauge plane17.15 mm / 0.675 in
Measured across the male crests16.6 mm to 17.3 mm
Basic minor diameter (mating female)15.62 mm / 0.615 in - computed from the 60 degree thread profile, not measured
Thread form60 degree flank angle, tapered, taper 1 in 16
SeatTapered thread, sealed on the thread
How the size is namedSized by trade size, which is neither the thread diameter nor the bore: 1/4 NPT measures about 0.54 in across the threads.

The two catalogues that publish this figure disagree at this size: one gives 4,000 psi and the other 3,000 psi at the same 4 to 1 design factor, an offset of one pipe size that runs through both tables. No working pressure is published here because there is no agreed one. Take it from the catalogue of the fitting you are actually installing.

Working pressure

No working pressure is published for this connection. Ratings for it vary by manufacturer, body material, port form and seal, and no two independent sources in the register give the same figure under conditions they both state. A generic number here would be a decoration, not a rating. Read the pressure from the catalogue of the fitting you are actually installing.

A fitting with two different ends is only ever as strong as the weaker end, and any rating assumes the fitting is undamaged, correctly assembled and within its temperature range. Pressure ratings are safety-relevant: confirm them against the manufacturer specification before the system is charged.

What it mates with

A female NPT or NPTF port of the same trade size. NPT to NPT needs a thread sealant or PTFE tape; NPTF to NPTF is designed to seal dry through crest and root interference, and NPT and NPTF assemble together.

A tapered thread seals by wedging, so it strips the port if it is over-tightened chasing a leak. Use sealant or PTFE tape, and stop at the published turns past finger tight rather than at a feel.

Do not mix these up

Fittings that measure like a 3/8 NPT

Every size below sits within 0.9 mm of this one across the male thread crests, which is inside what a worn or plated fitting and a hurried measurement produce together. Those marked same thread will screw together with this fitting.

Fittings in other standards whose male thread diameter is close to 3/8 NPT.
FittingThreadBasic major ØSeatWill it thread on?
-06 ORFS · ORFS (O-ring face seal)11/16-16 UNF17.46 mmO-ring face seal (flat face)No - 0.31 mm apart on diameter, and a different pitch
G 3/8 · BSPP (G)G 3/8 (19 TPI, 55 degree)16.66 mmParallel thread, sealed on a face gasketNo - 0.49 mm apart on diameter, and a different pitch
R 3/8 · BSPT (R)R 3/8 (19 TPI, 55 degree, taper 1:16)16.66 mmTapered thread, sealed on the threadNo - 0.49 mm apart on diameter, and a different pitch
-06 JIS 30 (G 3/8) · JIS 30 degree flare (BSPP)G 3/8 (19 TPI, 55 degree)16.66 mm30 degree flare cone or seatNo - 0.49 mm apart on diameter, and a different pitch
12L (M18 x 1.5) · Metric DIN 24 degreeM18 x 1.518 mm24 degree cone (DIN)No - 0.85 mm apart on diameter, and a different pitch
10S (M18 x 1.5) · Metric DIN 24 degreeM18 x 1.518 mm24 degree cone (DIN)No - 0.85 mm apart on diameter, and a different pitch

This list is computed from the published diameters in the database rather than kept as a hand-written list, so a correction to a table can never leave a stale warning behind.

Answers

Common questions about 3/8 NPT

What thread is a 3/8 NPT fitting?

3/8 NPT runs a 3/8-18 NPT thread: 18 TPI (1.411 mm pitch), basic major diameter 17.15 mm / 0.675 in. Published identification charts read 16.6 mm to 17.3 mm across the male crests, which is the figure a caliper produces.

How do I tell a 3/8 NPT apart from a fitting that measures the same?

Look at the sealing surface. No cone, no groove, no flat sealing face - just a thread that visibly narrows towards the end of the fitting. The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread. The thread is tapered and the flank angle is 60 degrees.

What does a 3/8 NPT screw into?

A female NPT or NPTF port of the same trade size. NPT to NPT needs a thread sealant or PTFE tape; NPTF to NPTF is designed to seal dry through crest and root interference, and NPT and NPTF assemble together.

What pressure is a 3/8 NPT rated for?

No working pressure is published here for this connection. Ratings vary by manufacturer, body material and seal, and no two independent sources in the register give the same figure under stated conditions for it. Read the rating from the catalogue of the fitting you are installing.

NPT / NPTF

Other sizes in this standard

1/16 NPT1/16 in trade size1/16-27 NPT1/8 NPT1/8 in trade size1/8-27 NPT1/4 NPT1/4 in trade size1/4-18 NPT1/2 NPT1/2 in trade size1/2-14 NPT3/4 NPT3/4 in trade size3/4-14 NPT1 NPT1 in trade size1-11 1/2 NPT1 1/4 NPT1 1/4 in trade size1 1/4-11 1/2 NPT1 1/2 NPT1 1/2 in trade size1 1/2-11 1/2 NPT2 NPT2 in trade size2-11 1/2 NPT

Where these figures come from

Dimensions are as defined by ASME B1.20.1, ASME B1.20.3. FittingKey is an independent reference and is not affiliated with, endorsed by or approved by any standards body or fitting manufacturer.

Every value on this page was checked against 6 independent published sources:

Which published source supplied each column, and what it gave.
ColumnSourcesWhat they gave
Basic major diameterasme-b1201, amesweb-npt, machiningdoctor-nptThe basic major diameter D is the outside diameter of the pipe, read directly from ASME B1.20.1-1983 Table 1 and matched to four decimal places by two independent reproductions of that table.
Measured male thread diametergerrard-id, gates-idchart, mcgill-idThree charts measuring a tapered thread at three different planes. The span is wide on purpose - a 1 in 16 taper means the reading depends on where the caliper sits.
Working pressuretompkins, brennan-pressureTwo catalogues, steel, 4 to 1 against minimum burst per SAE J514. They agree from 1/2 upward and disagree by one whole pipe size at 1/4 and 3/8, where no figure is published.

Sources disagree: NPTF working pressure at 1/4 and 3/8 nominal pipe size.

  • 5,000 psi at 1/4 and 4,000 psi at 3/8 (tompkins)
  • 4,000 psi at 1/4 and 3,000 psi at 3/8 (brennan-pressure)

Both tables state the same basis - steel, 4 to 1 against minimum burst - and agree exactly from 1/2 upward, so the gap is a one-size offset in one of them rather than a difference of design intent. No working pressure is published at 1/4 or 3/8. Take it from the catalogue of the fitting you are installing.