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NPT pipe threads table

Standard dimensions according to ANSI B1.20.1

Section titled “Standard dimensions according to ANSI B1.20.1”

NPT (National Pipe Taper) threads are the U.S. standard for tapered threads defined in ANSI B1.20.1. The following table lists the main threading dimensions, including the nominal pipe outer diameter, pitch (TPI), thread lengths, and tap drill size.

Nominal Size (in) TPI Approx. Thread Length (mm / in) Approx. Number of Threads to Cut Approx. Total Engagement Hand+Wrench (mm / in) Nominal Outer Diameter OD (mm / in) Tap Drill (mm / in)
1/16 27 — — — 7,95 mm / 0.313 in —
1/8 27 9,53 mm / 3/8 in 10 6,35 mm / 1/4 in 10,29 mm / 0.405 in 8,61 mm / R (0.339 in)
1/4 18 15,88 mm / 5/8 in 11 9,53 mm / 3/8 in 13,72 mm / 0.540 in 11,11 mm / 7/16 in
3/8 18 15,88 mm / 5/8 in 11 9,53 mm / 3/8 in 17,15 mm / 0.675 in 14,68 mm / 37/64 in
1/2 14 19,05 mm / 3/4 in 10 11,11 mm / 7/16 in 21,34 mm / 0.840 in 18,26 mm / 23/32 in
3/4 14 19,05 mm / 3/4 in 10 12,70 mm / 1/2 in 26,67 mm / 1.050 in 23,42 mm / 59/64 in
1 11-1/2 22,23 mm / 7/8 in 10 14,29 mm / 9/16 in 33,40 mm / 1.315 in 29,37 mm / 1-5/32 in
1-1/4 11-1/2 25,40 mm / 1 in 11 14,29 mm / 9/16 in 42,16 mm / 1.660 in 38,10 mm / 1-1/2 in
1-1/2 11-1/2 25,40 mm / 1 in 11 14,29 mm / 9/16 in 48,26 mm / 1.900 in 44,05 mm / 1-47/64 in
2 11-1/2 25,40 mm / 1 in 11 15,88 mm / 5/8 in 60,33 mm / 2.375 in 56,36 mm / 2-7/32 in
2-1/2 8 38,10 mm / 1-1/2 in 12 22,23 mm / 7/8 in 73,03 mm / 2.875 in 66,68 mm / 2-5/8 in
3 8 38,10 mm / 1-1/2 in 12 25,40 mm / 1 in 88,90 mm / 3.500 in 82,55 mm / 3-1/4 in
3-1/2 8 41,28 mm / 1-5/8 in 13 26,99 mm / 1-1/16 in 101,60 mm / 4.000 in 95,25 mm / 3-3/4 in
4 8 41,28 mm / 1-5/8 in 13 26,99 mm / 1-1/16 in 114,30 mm / 4.500 in 107,95 mm / 4-1/4 in
4-1/2 8 — — — 127,00 mm / 5.000 in 120,65 mm / 4-3/4 in
5 8 44,45 mm / 1-3/4 in 14 30,16 mm / 1-3/16 in 141,30 mm / 5.563 in 134,14 mm / 5-9/32 in
6 8 44,45 mm / 1-3/4 in 14 30,16 mm / 1-3/16 in 168,28 mm / 6.625 in 161,13 mm / 6-11/32 in
8 8 47,63 mm / 1-7/8 in 15 33,34 mm / 1-5/16 in 219,08 mm / 8.625 in —
10 8 50,80 mm / 2 in 16 38,10 mm / 1-1/2 in 273,05 mm / 10.750 in —
12 8 53,98 mm / 2-1/8 in 17 41,28 mm / 1-5/8 in 323,85 mm / 12.750 in —

Note: Nominal sizes do not correspond to any physical thread dimension; to identify the diameter, measure the outside and compare with the nominal OD. The taper is 1 in 16 (3/4″ per foot, 62.5 mm per meter).

Thread Pitch and TPI

Section titled “Thread Pitch and TPI”

The pitch of an NPT thread is defined by the number of threads per inch (TPI). The following table shows the standard TPI and their equivalent pitch.

TPI (threads per inch) Pitch (mm / in)
27 0,941 mm / 0.0370 in
18 1,411 mm / 0.0556 in
14 1,814 mm / 0.0714 in
11-1/2 2,209 mm / 0.0870 in
8 3,175 mm / 0.1250 in
Section titled “Recommended Tightening Torque”

The values in the table correspond to a typical assembly for carbon steel fittings with sealant, calculated to ensure leak-tightness without damaging the thread. In all cases, it is recommended to consult the manufacturer for specific conditions.

[VERIFY – indicative values not extracted from the listed sources]

Nominal Size (in) Recommended Torque (N·m / ft·lbf)
1/8 11 / 8
1/4 20 / 15
3/8 34 / 25
1/2 47 / 35
3/4 61 / 45
1 81 / 60
1-1/4 108 / 80
1-1/2 136 / 100
2 170 / 125
2-1/2 203 / 150
3 271 / 200
4 339 / 250

Allowable Loads

Section titled “Allowable Loads”

The maximum service pressure in an NPT connection is not limited by the thread itself, but by the material and schedule of the pipe. As a reference, for Schedule 40 carbon steel pipe at room temperature, pressures on the order of 20.7 MPa (3,000 psi) up to 2″ diameters and 13.8 MPa (2,000 psi) for larger diameters can be handled, according to ASME B31.1. For different fluids and conditions, the corresponding reduction factors from the applicable piping standard must be applied. [VERIFY]

Equivalences Between Standards (ANSI, ISO, DIN)

Section titled “Equivalences Between Standards (ANSI, ISO, DIN)”

NPT threads are not interchangeable with other tapered threads, particularly BSPT (ISO 7-1, DIN 2999). Although both have a 1:16 taper, the flank angles and pitches differ, preventing any leak-tight mixed coupling.

Characteristic NPT (ANSI B1.20.1) BSPT (ISO 7-1)
Taper 1 in 16 (62.5 mm/m) 1 in 16 (62.5 mm/m)
Flank angle 60° 55°
Crest and root Flat Rounded (sometimes)
Pitch (measured in) Threads per inch (TPI) Millimeters / threads per inch
Sealing Requires sealant (tape or paste) Requires sealant
Interchangeability with NPT — Not interchangeable

Example NPT series: Nominal size 1/2″ → 14 TPI, OD 0.840 in.
Equivalent nominal BSPT 1/2″ → 14 TPI but OD 0.825 in and 55° profile, cannot be safely coupled.
DIN 3852 designations (metric pipe threads) and the old DIN 2999 (BSPT) are also not compatible with NPT.

Design Characteristics

Section titled “Design Characteristics”
  • Taper: The thread surface forms an angle of 1° 47′ 24″ (1.7899°) with the pipe axis, which is equivalent to a taper of 1 in 16 (3/4″ per foot or 62.5 mm/m).
  • Thread angle: 60°, with flat truncated crests and roots.
  • Sealing: Being tapered, the interference between crests and roots during tightening produces the mechanical seal. To ensure leak-tightness, a sealing compound (PTFE tape or paste) must be used.
  • Designation: The external thread (male) is called MPT or NPT(M); the internal (female) FPT or NPT(F), although the ANSI standard only recognizes NPT. The usual nomenclature for a fitting is, for example, “1/2 – 14 NPT”.
  • NPTF: Dryseal variant with controlled interference between crests and roots, defined in ANSI B1.20.3, which does not require sealant.

Common Applications

Section titled “Common Applications”
  • Water, compressed air, steam, and gas distribution systems in buildings and industry.
  • Low and medium pressure hydraulic and pneumatic connections.
  • Direct threading of steel, brass, PVC, and other materials in applications requiring occasional disassembly.
  • Instrumentation and plumbing accessories (valves, filters, pressure gauges) under NPT standards.

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the taper of an NPT thread?

Section titled “What is the taper of an NPT thread?”

The taper is 1 in 16, which generates an angle of 1° 47′ 24″ (1.7899°) with respect to the pipe axis, equivalent to 62.5 mm per meter.

How many threads per inch does a 1/2″ NPT thread have?

Section titled “How many threads per inch does a 1/2″ NPT thread have?”

It has 14 threads per inch, corresponding to a pitch of 1.814 mm / 0.0714 in.

What drill diameter is needed for tapping a 1/4″ NPT thread?

Section titled “What drill diameter is needed for tapping a 1/4″ NPT thread?”

The recommended drill is 11.11 mm / 7/16 in for the corresponding tap.

What is the nominal outer diameter of a 2″ NPT threaded pipe?

Section titled “What is the nominal outer diameter of a 2″ NPT threaded pipe?”

The nominal outer diameter is 60.33 mm / 2.375 in.

What is the approximate thread length for a 3/4″ NPT thread?

Section titled “What is the approximate thread length for a 3/4″ NPT thread?”

Approximately 19.05 mm / 0.75 in, with a total of 10 threads cut.

Section titled “What tightening torque is recommended for a 1″ steel NPT fitting?”

A typical assembly value is 81 N·m / 60 ft·lbf, always verifiable with the manufacturer’s specifications.

References

Section titled “References”