Every discussion about pipe and fitting sizes starts in a state of confusion. That is because, unless you are a plumber, it is natural to assume that the 1-1/2” elbow you have in your hand measures one and one-half inches… somewhere. In fact, only plumbers are taught that one’s logical assumption is not true.

Like dimensional lumber, where a 2 X 4 does not measure 2” by 4” there is no dimension on a piece of pipe or plumbing where the actual measurement matches the nominal size.

There are also many different thread designs depending upon their purpose, history, the area of the world they were first developed and the standards authority that establishes the standards. Here are just a few examples:

• NPT:  National Pipe Thread Taper
• NPTF: Dryseal American National Standard Taper Pipe Thread (ANSI B1.20.3)?NPT threads may be referred to as MPT ('Male Pipe Thread'), MNPT, or NPT(M) for male (external) threads
• FPT ('Female Pipe Thread'), FNPT, or NPT(F) for female (internal) threads. Note that FPT and MPT are not authorized designations per the ANSI standard.
• ISO: Thread pitch is usually measured in millimeters. Threads have 55° taper angle versus 60° for NPT. The NPT root and crest configurations are also different from ISO.

Most actual plumbing parts aboard (with the excepeion of through hull fittings) are NPT and look like this:

Characteristics of NPT (also known as ANSI/ASME B1.20.1 Pipe Threads, General Purpose):

• Tapered thread 1° 72'
• 60° thread angle
• Pitch is measured in threads per inch
• Truncation of roots and crests are flatImportant notes: 

Important tidbits:

• For NPT threads a sealant compound or Teflon tape must be used for a leak-free seal. 
• Each thread size has a defined number of threads per inch - TPI, or pitch. For example, both 1/2" and 3/4" NPT pipe has 14 threads per inch, therefore both the TPI and OD of the thread are required for positive identification of thread size.
• NPT threads are not interchangeable with NPS - National Pipe Straight - threads.
• NPT threads may look similar to ISO 7/1 threads. However, ISO and NPT threads should not be mixed.