Tapered Thread for American Pipes (NPT)

ANSI / ASME B1.20.1 American-made national pipe tapered thread (NPT) is an acute angular shape with a 60° shank angle and is distributed in 1/16 of the taper on the conical tube. Although the taper of ISO R(PT) pipe tapered thread is also 1/16, but because the asan angle is 55°, it cannot be used with NPT thread.

Thread representation

NPT [Named Diameter]

Example

NPT ⅛

Thread Specifications

Thread Angle: 60° (American NPT)
Standard: ANSI/ASME B1.20.1
Taper: 1/16 (1.789°)
Application: Pipe connections

NPT Thread Specifications

Unit mm

Name of Thread	Threads per Inch	Pitch P	Major Diameter D, d	Median Diameter D₂, d₂	Minor Diameter D₁, d₁	Base Length L

Important Note

Base Length: Since the thread diameter on the cone surface will change with position, an imaginary base plane is defined as a reference for the major diameter, middle diameter, and minor diameter. The distance between this surface and the external thread end face is called Datum length.

FAQ

What is the difference between NPT and ISO R(PT) tapered threads?
NPT threads have a 60° thread angle and 1/16 taper following ANSI/ASME B1.20.1 standards. ISO R(PT) threads have a 55° angle with the same 1/16 taper. These differences make them incompatible despite similar appearance.
Why are NPT threads tapered instead of parallel?
The 1/16 taper (1.789°) creates a wedging action that provides a metal-to-metal seal as the threads are tightened. This eliminates the need for gaskets in many applications and creates a leak-proof connection for pressurized systems.
What does "Base Length" mean in NPT threading?
Base Length (datum length) is the distance from an imaginary reference plane to the end face of the external thread. Since thread diameters change along the taper, this reference plane provides a standard measurement point for major, pitch, and minor diameters.
How do I properly assemble NPT fittings?
Apply pipe thread sealant or PTFE tape to the male threads. Hand-tighten until snug, then use a wrench to tighten an additional 1-3 turns depending on size. Over-tightening can crack fittings or cause thread damage.
What are common applications for NPT threads?
NPT threads are widely used in plumbing, hydraulics, pneumatics, and industrial piping systems. Common applications include pipe fittings, valves, gauges, and any pressurized fluid connection where a reliable seal is required.
Can NPT threads be reused after disassembly?
NPT threads can be reused, but the sealing effectiveness may be reduced. Clean old sealant thoroughly and apply fresh sealant or tape. Inspect threads for damage before reuse, as the wedging action can cause wear over multiple assembly cycles.