NPT Fittings

NPT Fittings

NPT stands for National Pipe Tapered thread. It is also sometimes called National Pipe Thread, NPTF (F for Fuel), American Standard Taper, or simply “pipe”. Most other plumbing fittings have straight or parallel threads, but NPT fittings have a tapered thread profile. The outside diameter of the male thread tapers down from the base, so the end of the fitting has a smaller OD than the base of the fitting. The matching female thread has a larger OD at the opening, tapering down as it goes in.

NPT fitting sizes refer to the nominal ID of a standard pipe. This is where the confusion comes in. Most plumbing fittings are named for the thread size, but NPT sizes are not. An NPT fitting measuring 3/8″ OD thread is actually a 1/8 NPT, not 3/8 NPT. Use the chart below to positively identify NPT fitting sizes.

NPT Size Identification

Male Thread OD
at middle of taper
Threads per Inch
Female Thread ID
at opening
NPT Size