NPT sample (tapered threads)

National Pipe Threads (NPT) have tapered threads. These are the most common threads used for general purposes. NPT threads are designed with a 60 degree thread angle, and are used for joining and sealing pipe to fittings in low pressure air or liquids and also mechanical applications. The tapered thread is 3/4" over one foot of length. Tapered threads are deeper at the end of the pipe and are increasingly shallower the further they are from the end of the pipe. The taper on the pipe only allows the pipe to screw inside the fitting until it is forced to stop because of the taper. The distance the pipe can be screwed into the fitting is specified by the ANSI standard. After tightening with a wrench the threads may have slight spaces between the pipe and fitting which could cause a leak so a pipe sealant must be used to ensure any gaps are filled.

The Dry-seal thread (NPTF) also have tapered threads. NPTF threads are used when the application is such that pipe sealing compounds may fail due to higher heat or pressure than normal NPT threads can withstand. The threads are designed to seal mechanically by slightly, but sufficiently, crushing the threads when tightened with a wrench. This allows for joining the pipe and fitting without sealants.

The NPT and NPTF threads are interchangeable with sealants such as PTFE tape or suitable pipe joint compounds. None of the other thread standards are interchangeable. Female NPT threads can be designated as "FPT" and male NPT threads can be designated as "MPT."

National Standard Free-Fitting Straight Mechanical Pipe Threads (NPSM) have straight threads which are only used for joining. A washer or gasket is needed to seal this type of threaded connection.

There are also three less common thread types used in the plumbing industry. The Garden Hose Thread (GHT) and the Fire Hose Thread (NST) have course threads. The seal is made with a gasket or washer and are used mainly for attaching (joining) hoses to valves quickly, without the use of a wrench. The British Standard Taper Pipe Thread (BSPT) has a 55 degree thread angle (NPT are 60 degree) and is used internationally as a standard thread for joining steel pipes.