Pipe Threads / NPT
Overview
If you have ever worked with pipe fittings or threaded pipe in the past, you're probably familiar with the sizing of the threads and the OD/ID of each. For most, pipe threads can be rather confusing and make it difficult to know what to buy. For instance, a 1/4" NPT pipe thread will measure out to ~0.540" on the outside of the thread. What gives? I thought this was a 1/4" fitting.
In most cases, measuring one of these fittings with a tape measure will leave you scratching your head and more confused than when you started. This article will go over the different nuances between sizing NPT threads and help you determine what size fitting or port you currently have.
Determining the Thread Size with a Standard Measurement
| Pipe Size | Outside Diameter | Threads Per Inch |
| 1/8" NPT | 0.405" | 27 |
| 1/4" NPT | 0.540" | 18 |
| 3/8" NPT | 0.675" | 18 |
| 1/2" NPT | 0.840" | 14 |
| 3/4" NPT | 1.050" | 14 |
With a standard tape measure, a 1/4" NPT fitting should measure out to approximately 0.540". You can refer to this chart for any male pipe thread. Female pipe threads will measure a little different, but not far off from these.