How to measure npt pipe fittings?

Nov 19, 2025

What Is NPT Pipe Fittings?

NPT stands for National Pipe Taper, known in Chinese as the "American National Tapered Pipe Thread." The threads of this type of fitting are tapered, resembling a pin, with the diameter gradually increasing from one end to the other. It creates a seal through the taper of the threads and requires the use of Teflon tape or pipe thread sealant to achieve an effective seal. This NPT fitting is widely used in various fluid (water, gas, oil) pipeline systems across North America.

Meaning of “Tapered Thread”

Tapered thread refers to a tapered thread, the core characteristic of which is that the thread diameter gradually decreases along its length.

In NPT (Non-Pipeline Technology), the taper ratio is: 1 inch of diameter reduction per 16 inches of thread length. That is, the diameter decreases by 1/16 inch per inch of thread length. This structure allows the thread to create a wedge-like effect when tightened, forming a seal through the interference fit between the metal parts.

Features and Advantages of NPT Pipe Fittings

Excellent Sealing Performance:The tapered design ensures a tight metal-to-metal contact when tightened, making the self-sealing capability of NPT fittings its biggest advantage.

High Pressure Resistance:Due to the clamping force created at the connection point by its tapered structure, NPT fittings can withstand medium to high pressure conditions.

Low Cost:NPT fittings have a very simple structure, without additional seals or complex mechanical mechanisms, which reduces the requirements for manufacturing capabilities.

Wide Range of Sizes and Materials

NPT pipe fittings can be manufactured from a variety of materials to meet different environmental needs

Brass: Commonly used in pneumatic, hydraulic, and low-pressure systems

Stainless Steel: Suitable for corrosive environments, sanitary industries, and high-pressure conditions

Carbon Steel: The first choice for industrial hydraulic systems, with strong pressure resistance

Plastic (PVC/CPVC): Chemical and low-pressure water treatment systems

Bronze, Copper, etc.: Specific applications such as marine and HVAC.

Key Differences Between Tapered Threads (NPT) and Straight Threads

Characteristic	NPT	NPS / Straight Threads (BSPP, G-Thread, etc.)
Thread Shape	Tapered	Straight / Cylindrical
Sealing Method	Seals on the threads themselves (often enhanced with PTFE tape or thread sealant)	Does not self-seal; requires O-rings, gaskets, or metal-to-metal sealing surfaces
Pressure Capability	Suitable for medium to high-pressure systems	Better suited for low-pressure or applications requiring frequent disassembly
Tightening Behavior	Tightens progressively due to wedging and interference fit	Tightening to the end does not create a seal
Need for Sealant	Typically required	Usually not required (sealed by O-ring or gasket)

How To Measure NPT Pipe Fittings?

Measuring NPT fittings primarily involves determining their nominal diameter rather than directly measuring the physical diameter of the threads. Here are the simple measurement steps:

Measure the outer diameter: For male threads, use calipers to measure the major diameter of the threads.

Refer to a size chart: Compare the measured outer diameter with a standard NPT size reference chart to identify the corresponding nominal diameter.

A quick estimation reference:

The major diameter of a 1/8-inch NPT male thread is approximately 10.3 mm.

The major diameter of a 1/4-inch NPT male thread is approximately 13.7 mm.

The major diameter of a 1/2-inch NPT male thread is approximately 21.0 mm.

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