What is NPT Thread?

NPT stands for National Pipe Taper thread. It’s a tapered thread standard commonly used in North America for pipe and fitting connections in various applications, including:

Key features of NPT threads:

  • Tapered threads: The threads have a slight taper, meaning they gradually get narrower towards the end. This taper creates interference when the threads are tightened, forming a seal.
  • Requires sealant: NPT threads typically require a sealant, such as Teflon tape or pipe dope, to ensure a leakproof connection.
  • Designation: NPT threads are designated by their nominal size in inches (e.g., 1/4″ NPT, 1/2″ NPT).
  • Standard: The technical standard for NPT threads is defined by ANSI/ASME B1.20.1.

Advantages of NPT threads:

  • Common standard: Widely used in North America, making it easy to find compatible components.
  • Strong and durable: The tapered design provides a strong and secure connection.
  • Good for high-pressure applications: Can withstand significant pressure without leaks.

Applications of NPT threads:

  • Plumbing: Widely used in water and gas pipe systems.
  • Hydraulics: Used in hydraulic systems, but often in conjunction with sealants for leakproof connections.
  • Compressed air: Used in compressed air systems.
  • Fuel lines: Used in fuel lines for vehicles and machinery.
  • General piping: Employed in various other piping applications.

Important considerations:

  • Sealant: Always use a sealant when making NPT connections to prevent leaks.
  • Compatibility: NPT threads are not directly compatible with parallel threads like BSPP or metric parallel threads. Adaptors are required if you need to connect them.
  • Overtightening: Avoid overtightening NPT fittings as it can damage the threads and create leaks.

Here are the key dimensions of NPT threads:

drawing NPTF


  • The taper rate for all NPT threads is 1 inch of diameter in 16 inches of length (3/4 inch per foot or 62.5 millimeters per meter). This means the threads gradually get narrower by 1 inch for every 16 inches of length. it is about 4°.

Threads per inch (TPI):

  • The number of threads per inch varies depending on the nominal size of the thread. Here are common examples:

Major diameter:

  • The largest diameter of the thread, measured at the base of the thread.

Minor diameter:

  • The smallest diameter of the thread, measured at the crest of the thread.


  • The distance between adjacent threads, measured along the axis.

Thread angle:

  • The angle between the flanks of the thread, which is typically 60 degrees for NPT threads.

Here’s a table summarizing the dimensions for some common NPT thread sizes:

Nominal Size (inches) Major Diameter (inches) Minor Diameter (inches) Pitch (threads per inch)
1/8 0.405 0.354 27
1/4 0.540 0.478 18
3/8 0.675 0.609 18
1/2 0.840 0.758 14
3/4 1.050 0.965 14
1 1.315 1.215 11.5

Additional notes:

  • The taper rate is crucial for creating a tight seal when NPT threads are tightened.
  • The threads per inch determine how fine or coarse the threads are.
  • The major and minor diameters are important for compatibility with fittings and other components.
  • The pitch ensures consistent thread engagement.
  • The thread angle is a standard feature of NPT threads.

Always refer to ANSI/ASME B1.20.1 for the full technical specifications of NPT threads.

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