NPT thread - chart, dimensions, how to measure

The main image of article about NPT threads

What is an NPT thread?

An NPT thread is an inch tapered thread with a 60-degree profile angle, a 'V' shape and a flattened crest. The abbreviation itself refers to 'National Pipe Tapered Thread'.

It appeared in 1864 in America, where it was established as a national standard by William Sellers.

It is worth mentioning here that there is another name for NPT - the Briggs tapered pipe thread. Therefore, if you were to come across one, know that it is the same thread.

On the market you can also find the NPS thread, which is also an American thread, but cylindrical (not tapered). However, it is much less commonly used.

Female NPT thread and Male NPT thread

What is the application of the NPT thread?

The main application of the NPT thread is a joint of a sealing nature in various hydraulic, pneumatic, etc. installations. Due to the origin of this thread type, you will most often encounter it in Canada and the United States, but it is popular all over the world.

The joint works on the principle that the external thread pulls the internal valve, so that the system maintains the correct pressure.

What are the dimensions of NPT threads? NPT thread chart

Below, we have collected the dimensions of NPT threads:

Thread size [inch] Number of coils per inch Thread pitch in [mm] Outer Diameter [mm]
1/8 27 0.94082 10.29
3/8 18 1.41122 13.72
1/2 18 1.41122 17.15
3/4 14 1.81432 21.34
1 14 1.81432 26.67
1 1/4 11 1/2 2.20878 33.4
1 1/2 11 1/2 2.20878 42.16
2 11 1/2 2.20878 48.26
2 1/2 11 1/2 2.20878 60.33
3 8 3.175 73.03
4 8 3.175 88.9
4 8 3.175 114.3

How to measure a NPT thread?

You can measure NPT threads with a calliper.

When it comes to the outer diameter, simply grip the thread at its widest point with the jaws of the tool and read the result. You will measure the inner diameter similarly, but in this case use the smaller jaws of the calliper.

The measurement of the thread pitch is done a bit differently. In this case, put the jaws of the calliper parallel to the thread so that each jaw is adjacent to a separate of the thread crest.

The best way is to do this over a space of 5 crests, and divide the result by 4 (why 4? Because you are dividing by the number of roots between the crests). The resulting number is the pitch of the given thread.

Compare it with the chart and see which NPT thread you need!

Photo of Bartosz Kułakowski - CEO of Hosetech sp. z o. o.

Articles author

Bartosz Kułakowski

CEO of Hosetech Sp. z o.o.

Bartosz Kulakowski is an industrial hoses and couplings specialist with over 10 years of experience. Bartosz has been present in the technical industry since 2013. He gained experience as a technical and commercial advisor in the sector of plastic conveyor belts, steel structures, industrial hoses, and connectors. Since 2016, he has specialized exclusively in hoses and connectors. In 2019 he opened his own business under the HOSETECH Bartosz Kulakowski brand and since July 2022 he has been the CEO of the capital company HOSETECH Sp z o. o. (LLC).


What is an NPT thread?

What is the application of the NPT thread?

What are the dimensions of NPT threads? NPT thread chart

How to measure a NPT thread?