Every few months to a year I end up needing to do some plumbing. Each time I forget what I’ve learned previously and end up spending a couple hours at Lowes and buying 20+ adapters to find the right one. By the time I get it fixed, I’ve built up a decent bit of context around plumbing pipes and fittings, only to forget it before the next issue.

So, coming off the heels of yet another plumbing issue, this braindump is my reference for next time.

Threaded Fitting Types

NPT

The most common thread type in the US. Contains tapered threads that interlock to form a seal when tightened.

• NPT - National Pipe Thread / National Pipe Tapered
• IPT - Iron Pipe Thread
• MIP - Male Iron Pipe
• MPT - Male Pipe Thread
• FIP - Female Iron Pipe
• FPT - Female Pipe Thread

All of these refer to the same type thread.

NPT = IPT = MIP = MPT = FIP = FPT (of course barring the male/female differences)

⚠️ If you take a male NPT fitting to a hardware store and try it out on a female NPS/Compression fitting, it will look like it works, but won’t create a proper seal. Be sure to test NPT fittings on other NPT fittings.

NPS & Compression

NPS or National Pipe Straight fittings contain threads that are straight as opposed to tapered like NPT. In plumbing, compression fittings are NPS. If you see a compression nut, or ferrule, on a fitting, it’s compression. Compression fittings are also commonly sealed with a gasket.

⚠️ Sometimes you’ll see the marking NPS which may or may not refer to a thread type. It could also refer to the pipe’s size - Nominal Pipe Size.

⚠️ If you take a female Compression fitting to a hardware store and try it out on a male NPT fitting, it will look like it works, but won’t create a proper seal. Be sure to test Compression fittings on other Compresson fittings.

Sizing

Compression fittings generally use the outer diameter of the pipe they are connecting (I.e. the non-threaded size) as the size on one end and the nominal pipe size of the threaded other end. E.g. If you are connecting an 1/2” OD pipe to a 1/2 nominal pipe size threaded pipe with a compression fitting, you’d use a 1/2” x 1/2” compression fitting.