ANSI B1.20.1 – NPT – American National Standard Taper Pipe Threads

ANSI B1.20.1 – NPT – American National Standard Taper Pipe Threads

  • NPT – National Pipe Thread Tapered – is a U.S. standard for tapered threads used to join pipes and fittings is defined by ANSI B1.20.1 Pipe Threads, General Purpose, Inch
  • NPTF – National Pipe Thread Tapered Fine – also called Dryseal American National Standard Taper Pipe Thread -is a U.S. standard for tapered threads used to join pipes and fittings is defined by ANSI B1.20.3

For NPT threads a sealant compound or Polytetrafluoroethylene (PTFE) tape must be used for a leak-free seal. For NPTF no sealant is needed for a leak tight connection. NPTF threads are the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.

NPT threads may be referred to as MPT (‘Male Pipe Thread’), MNPT, or NPT(M) for male (external) threads, and FPT (‘Female Pipe Thread’), FNPT, or NPT(F) for female (internal) threads. Note that FPT and MPT are not authorized designations per the ANSI standard.

Characteristics of NPT (also known as ANSI/ASME B1.20.1 Pipe Threads, General Purpose):

  • the angle between the taper and the center axis of the pipe is  1o 47′ 24” (1.7899o)
  • truncation of roots and crests are flat
  • 60o thread angle
  • pitch is measured in threads per inch – TPI

Note! Pipe sizes do not refer to any physical dimensions. The outside diameter of a pipe or fitting must be measured and compared to a table for size identification. A 3/4″ NPT pipe thread has an outside diameter – OD – of 1.050 inches. Each thread size has a defined number of threads per inch – TPI, or pitch. The 3/4″ NPT pipe thread has 14 threads per inch. Both the TPI and OD of the thread are required for positive identification of thread size because more than one size have the same TPI.

NPT – American Standard Pipe Thread Taper to Inches Convert Chart

Pipe Size
TPIApproximate Length of Thread (inches)Approximate Number of Threads to be CutApproximate Total thread Makeup, Hand and Wrench (inches)Nominal Outside Diameter Pipe
Tap Drill
2-1/2″81 1/2127/82.8752-5/8
3″81 1/21213.5003-1/4
3-1/2″81 5/8131 1/164.0003-3/4
4″81 5/8131 1/164.5004-1/4
4 1/2″85.0004-3/4
5″81 3/4141 3/165.5635-9/32
6″81 3/4141 3/166.6256-11/32
8″81 7/8151 5/168.625 
10″82161 1/210.750 
12″82 1/8171 5/812.750 

1) The taper rate is 1 in 16 inch (3/4 inch in a foot or 62.5 mm in a meter)

  • TPI = threads per inch
  • 1 inch = 25.4 mm

NPT threads are not interchangeable with NPS – National Pipe Straight – threads.

Example – Typical Designation of NPT thread:

  • 1⁄8–27 NPT

NPT threads may look similar to ISO 7-1 threads. However, ISO and NPT threads should not be mixed. ISO threads have 55o taper angle versus 60o for NPT. The NPT root and crest configurations are also different from ISO. For ISO threads pitch is usually measured in millimeters (may be expressed in Inch). The pitch is different.

  • NPS – American National Standard Straight Pipe Thread

Note that both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight (parallel). Both threads have a 60° included angle and flat peaks and valleys.


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