To determine the size of a threaded air port in an air tool handle, air cylinder, an valve, or the female thread in any compressed air component measure the diameter of the hole in inches. Compare that result to the fractional sizes on the chart to help guide you to the right male NPT fitting size to use.
As mentioned, NPT is an acronym for National Pipe Taper, an industry dimensional standard. Recognize that the threads will connect, but just that they will connect together doesn't mean that all of the fittings will necessarily look the same.


What do you need to know when you are trying to determine what fitting to use and where to use it? You need to identify the...

• Port size / fitting thread size into which the male thread is screwed
• Size of tube or hose that you want to attach to the fitting; and note, tube and hose do NOT connect the same way
• Style of fitting; whether it is a straight, elbow, Tee , run Tee etc.)
• Way the hose or tube is to be connected to the fitting

In NPT sizes, you will commonly see the following;

• 10-32
• 1/8 NPT
• 1/4 NPT
• 3/8 NPT
• 1/2 NPT

There are certainly much larger NPT sizes. Those shown above are the commonest fittings in industrial compressed air applications.


The NPT type thread seals by penetrating the thread boss until tight and sufficient threads on the fitting are interlaced with sufficient threads in the boss to generate a seal.