Air Fittings - NPT National Pipe Thread - Guide

NPT - National Pipe Thread

We are often asked what size a thread is and above all why e.g. not 1/2" NPT is 12.7mm (0.5").
Here we will try to explain this and finally there will also be a table with the outer diameter of a MNPT(Male) in the different sizes.

NPT is originally a thread that was used for to assemble pipes where liquids, gas, steam and hydraulics were transported. This was done with adapters at the ends that linked 2 pipes together. By then making the thread conical, it could seal itself by simply tightening it. So no flat surface with any seal like a copper gasket or o-ring was required, but the thread still seals. "Sealing" should probably be said, because you realized early on that it is difficult to get it completely tight and have therefore experimented with different angles on the threads, but you have realized that you need thread sealing to get it perfect. Therefore, there is almost always thread tape on this type of coupling.

Nowadays, we mostly come across this type of coupling when we are building air suspension or other pneumatic systems in cars, etc.

Why then is not a thread that is 1/2"-14 NPT 12.7 mm on the outside?
The reason for that is that you start from the size of the pipe. An adapter that is 1/2" is therefore made for a pipe or hose that is 1/2" on the outside. In order for the pipe to fit in the adapter, it needs therefore be larger than this (To accommodate some material). 1/2"-14 NPT is therefore 21.3mm external on a male thread (MNPT).

Female(Female): FNPT
Male(Male): MNPT

Below is a table that makes it easy to measure what type of adapter you need for your bellows or hose.

Thread Size (Inches)	Number of Threads per Inch	Outside Diameter (Inches)	Outer Diameter (mm)


1/18"	27	0.3125	7.938
1/8"	27	0.405	10.287
1/4"	18	0.540	13.716
3/8"	18	0.675	17.145
1/2"	14	0.840	21.336
3/4"	14	1.050	26.670
1"	11.5	1.315	33.401
1-1/4"	11.5	1.660	42.164
1-1/2"	11.5	1.900	48.260
2"	11.5	2.375	60.325