Pipe threads are commonly used in plumbing, gas, and industrial systems - but different thread standards are used around the world. That is why a 1-inch thread in the USA may differ, for example, from a 1-inch thread in Europe - and as a result, hydraulic devices from these countries may be incompatible in this respect. In Europe, the BSP thread standard is commonly used, and when the thread type is not specified, but only the size is given (e.g. 1", 1 1/4", 3/4", etc.), it most likely refers to the thread size according to the BSP standard. However, in the USA, Canada, and outside Europe, the NPT thread is often the most popular. This does not mean that NPT is completely unheard of in Europe, although it is much less common. It has specific applications.
In this article, you will learn exactly what an NPT thread is and how it differs from other types of pipe threads. In this article, we explain where it is used and what its characteristic features are.
NPT thread (National Pipe Thread) is a pipe thread standard used mainly in the United States, Canada, and in countries that follow American technical standards. It is a tapered thread, which means that its diameter decreases along the length of the thread.
Thanks to its tapered shape, the NPT thread creates a very tight connection when tightened. The seal is formed by pressing the thread surfaces against each other. In practice, PTFE tape or special sealing compounds are also often used.
The most important feature of the NPT thread is its tapered design. The taper is 1:16, which means that the thread diameter changes by 1 unit for every 16 units of length. This is a simple distinction, although in terms of tapered structure it may somewhat resemble a sub-variant of the BSP thread, namely BSPT taper thread (while still not being compatible with it).
The NPT thread also has a characteristic thread angle of 60°. This allows it to work well with components made according to the same standard. Unlike European pipe threads, the seal does not occur on the end face, but directly on the thread.
It is also worth knowing that NPT threads come in various sizes, such as 1/4", 3/8", 1/2", 3/4", or 1". These sizes refer to the nominal pipe diameter, not the actual thread diameter.
☑️✅ The NPT thread size naming system is therefore imperial - the same as in the case of BSP. Therefore, it should be remembered that the same thing does not apply here. A 3/4-inch NPT thread and a 3/4-inch BSP thread will not be compatible with each other.
You can find the NPT thread chart below; it presents the exact dimensions of threads in this standard, along with explanations below the table.
| NPT thread size | Outside thread diameter (mm) |
Threads per inch (TPI) |
Thread pitch (mm) |
|---|---|---|---|
| 1/16" NPT | 7.90 | 27 | 0.94 |
| 1/8" NPT | 10.29 | 27 | 0.94 |
| 1/4" NPT | 13.62 | 18 | 1.41 |
| 3/8" NPT | 17.06 | 18 | 1.41 |
| 1/2" NPT | 21.34 | 14 | 1.81 |
| 3/4" NPT | 26.67 | 14 | 1.81 |
| 1" NPT | 33.40 | 11.5 | 2.21 |
| 1 1/4" NPT | 42.16 | 11.5 | 2.21 |
| 1 1/2" NPT | 48.26 | 11.5 | 2.21 |
| 2" NPT | 60.33 | 11.5 | 2.21 |
Explanations:
The NPT thread is often confused with other pipe thread standards, especially the BSP thread used in Europe. However, the differences between them are significant.
BSP thread (British Standard Pipe) comes in two variants: BSPP (parallel thread) and BSPT (tapered thread). Although BSPT is also tapered, its geometry is different from that of NPT. Among other things, it differs in the thread angle, which is 55°. Therefore, one may try to connect them - but sealing problems will occur.
For this reason, components with NPT threads should not be connected to BSP components. Although they can sometimes be screwed together mechanically, such a connection will not be fully tight and may lead to leaks.
NPT thread is very widely used in industrial and technical installations. It is most commonly found in:
Due to the popularity of American technical equipment, NPT thread also appears in many products imported into Europe, for example in UV lamps for domestic water treatment originating from the American market (European manufacturers tend to use BSP instead), or in specialist filters.
Identifying an NPT thread can be difficult, especially when comparing it with other pipe threads. However, there are several features that help with identification.
First, the NPT thread is clearly tapered. This means that its diameter gradually decreases. Second, it has a 60° thread angle. Third, it is most often marked with the abbreviation NPT in technical documentation or on the packaging of the component - if such documentation is available.
In professional applications, special thread gauges and measuring instruments are used to identify threads.
The tapered design of the NPT thread means that when components are tightened, the threads press very strongly against each other. As a result, a highly leak-tight connection is created.
The additional use of PTFE tape or sealing compound fills the micro-gaps between the threads. Thanks to this, the connection is resistant to leaks even under high pressure.
For this reason, NPT thread is often used in installations that require reliability and durable connections - and for this reason, over time it displaced the BSP standard originating from Great Britain in the USA.
NPT thread is one of the most important pipe thread standards used in engineering. Its tapered design makes it possible to achieve very tight connections, which makes it a popular solution in water, gas, and industrial installations.
Although BSP threads are more commonly used in Europe, NPT thread still appears in many devices imported from North America. Therefore, knowledge of its features and its differences from other standards is very useful during the installation and servicing of systems. In addition, it is worth remembering that there are also other, rarer standards intended for specific applications, for example gas-related ones, such as SAE threads (also of American origin).
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