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National Pipe Thread Taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. In contrast to straight threads that are found on a bolt, a taper thread will pull tight and therefore make a fluid-tight seal. In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864. 
Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. These threads are now used in materials other than steel and brass, including PTFE, PVC, nylon, bronze, and cast iron.
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft or 62.5 mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr free and lubricated using a lubricating material like lubricating paste or tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.
Commonly used sizes are 1⁄8, 1⁄4, 3⁄8, 1⁄2, 3⁄4, 1, 1 1⁄4, 1 1⁄2, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Sizes smaller than 1⁄8 inch are occasionally used for compressed air, while sizes larger than 2 inches are uncommon, due to the use of alternative methods of joining that are used with these larger sizes.
Pipe threads are different from machine screw and bolt threads. Those are designated NC (national coarse) and NF (national fine.) The biggest difference is the taper on pipe threads.
The taper rate for all NPT threads is 1 in 16 (3⁄4 inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance. The angle between the taper and the center axis of the pipe is tan−1(1⁄32) = 1.7899° = 1° 47′ 24″.
Nominal Pipe Size (NPS) is loosely related to the inside diameter of Schedule 40 pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Pipe of other schedules with a certain NPS has different wall thickness, but the same outside diameter and thread profile as Schedule 40, so the inside diameter of the pipe is therefore different from the NPS.
NPTE and NPS threads have a 60° included angle and have a Sellers thread form (flattened peaks and valleys).
A semi-compatible variant called National Pipe Taper Fuel (NPTF), also called Dryseal American National Standard Taper Pipe Thread, defined by ANSI B1.20.3, is designed to provide a more leak-free seal without the use of teflon tape or other sealant compound. NPTF threads are the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.
|Pipe Size (inches)||TPI|
Threads per Inch
(inches or mm)
Length of Thread
(inches or mm)
|Approximate Number of|
Threads to be Cut
Total thread Makeup,
Hand and Wrench
(inches or mm)
Pipe Diameter OD
(inches or mm)
(inches or mm)
|1⁄16||27||0.03703704 in or 0.9407 mm||0.313 in or 7.950 mm|
|1⁄8||27||0.03703704 in or 0.9407 mm||3⁄8 in or 9.525 mm||10||1⁄4 in or 6.350 mm||0.405 in or 10.287 mm||R|
0.339 in or 8.6106 mm
|1⁄4||18||0.05555555 in or 1.4111 mm||5⁄8 in or 15.875 mm||11||3⁄8 in or 9.525 mm||0.540 in or 13.716 mm||7⁄16 in or 11.113 mm|
|3⁄8||18||0.05555555 in or 1.4111 mm||5⁄8 in or 15.875 mm||11||3⁄8 in or 9.525 mm||0.675 in or 17.145 mm||37⁄64 in or 14.684 mm|
|1⁄2||14||0.07142857 in or 1.8143 mm||3⁄4 in or 19.0500 mm||10||7⁄16 in or 11.1125 mm||0.840 in or 21.3360 mm||23⁄32 in or 18.2563 mm|
|3⁄4||14||0.07142857 in or 1.8143 mm||3⁄4 in or 19.0500 mm||10||1⁄2 in or 12.7000 mm||1.050 in or 26.6700 mm||59⁄64 in or 23.4156 mm|
|1||11-1/2||0.08695652 in or 2.2087 mm||7⁄8 in or 22.2250 mm||10||9⁄16 in or 14.2875 mm||1.315 in or 33.4010 mm||1 5⁄32 in or 29.3688 mm|
|1 1⁄4||11-1/2||0.08695652 in or 2.2087 mm||1 in or 25.4000 mm||11||9⁄16 in or 14.2875 mm||1.660 in or 42.1640 mm||1 1⁄2 in or 38.1000 mm|
|1 1⁄2||11-1/2||0.08695652 in or 2.2087 mm||1 in or 25.4000 mm||11||9⁄16 in or 14.2875 mm||1.900 in or 48.2600 mm||1 47⁄64 in or 44.0531 mm|
|2||11-1/2||0.08695652 in or 2.2087 mm||1 in or 25.4000 mm||11||5⁄8 in or 15.8750 mm||2.375 in or 60.3250 mm||2 7⁄32 in or 56.3563 mm|
|2 1⁄2||8||0.125 in or 3.1750 mm||1 1⁄2 in or 38.1000 mm||12||7⁄8 in or 22.2250 mm||2.875 in or 73.0250 mm||2 5⁄8 in or 66.6750 mm|
|3||8||0.125 in or 3.1750 mm||1 1⁄2 in or 38.1000 mm||12||1 in or 25.4000 mm||3.500 in or 88.9000 mm||3 1⁄4 in or 82.5500 mm|
|3 1⁄2||8||0.125 in or 3.1750 mm||15⁄8 in or 15.8750 mm||13||1 1⁄16 in or 26.9875 mm||4.000 in or 101.6000 mm||3 3⁄4 in or 95.2500 mm|
|4||8||0.125 in or 3.1750 mm||1 5⁄8 in or 41.2750 mm||13||1 1⁄16 in or 26.9875 mm||4.500 in or 114.3000 mm||4 1⁄4 in or 107.9500 mm|
|4 1⁄2||8||0.125 in or 3.1750 mm||5.000 in or 127.0000 mm||4 3⁄4 in or 120.6500 mm|
|5||8||0.125 in or 3.1750 mm||1 3⁄4 in or 44.4500 mm||14||1 3⁄16 in or 30.1625 mm||5.563 in or 141.3002 mm||5 9⁄32 in or 134.1438 mm|
|6||8||0.125 in or 3.1750 mm||1 3⁄4 in or 44.4500 mm||14||1 3⁄16 in or 30.1625 mm||6.625 in or 168.2750 mm||6 11⁄32 in or 161.1313 mm|
|8||8||0.125 in or 3.1750 mm||1 7⁄8 in or 47.6250 mm||15||1 5⁄16 in or 33.3375 mm||8.625 in or 219.0750 mm|
|10||8||0.125 in or 3.1750 mm||2 in or 50.8000 mm||16||1 1⁄2 in or 38.1000 mm||10.750 in or 273.0500 mm|
|12||8||0.125 in or 3.1750 mm||2 1⁄8 in or 53.9750 mm||17||1 5⁄8 in or 41.2750 mm||12.750 in or 323.8500 mm|
|14||8||0.125 in or 3.1750 mm||14.000 in or 355.6000 mm|
|16||8||0.125 in or 3.1750 mm||16.000 in or 406.4000 mm|
|18||8||0.125 in or 3.1750 mm||18.000 in (457.2000 mm)|
|20||8||0.125 in or 3.1750 mm||20.000 in (508.0000 mm)|
|24||8||0.125 in or 3.1750 mm||24.000 in (609.6000 mm)|