NPT stands for National Pipe Thread, which is a U.S. standard for measuring tapered threads on threaded pipes and fittings. The thread body is tapered, which means that a larger diameter keeps compressing into a smaller diameter and finally forms a seal. NPT fittings should be burr-free and lubricated using a lubricant like lubricating paste or thread seal tape to limit corrosion on the threads, which can make future disassembly nearly impossible. NPT has a tapered male and female thread that seals with Teflon tape or jointing compound. NPT threads are not interchangeable with NPS (National Pipe Straight) threads. The taper rate for all NPT threads is 1/16—3/4 in. per foot (62.5 mm per m) . The outside diameter (OD) of a pipe or fitting must also be measured; both the TPI and OD are required for accurate identification of thread size because more than one size can have the same TPI. NPT threads were initially designed for water pipe plumbing, not for hydraulic systems, and are not recommended for high-pressure applications as they tend to leak more than any other style of connection.

Note: NPT can also refer to the Treaty on the Non-Proliferation of Nuclear Weapons, which is an international treaty whose objective is to prevent the spread of nuclear weapons and weapons technology, to promote cooperation in the peaceful uses of nuclear energy, and to promote nuclear disarmament.