IPT

Iron pipe thread, or IPT, is a tapered screw thread form used almost exclusively on steel and iron pipe fittings, valves, and connectors. The thread itself is cut or rolled onto the outside of male fittings and the inside of female sockets, allowing them to seal tightly when screwed together. IPT is defined by ANSI B2.1 and is sometimes called National Pipe Taper or NPT when applied to non-iron materials, though IPT specifically denotes the thread on ferrous metal pipe.

The taper is the defining feature: threads slope inward at roughly 0.75 degrees per side, so tightening naturally forces the fitting deeper into the socket and compresses the thread flanks against each other. This mechanical interference creates a seal without requiring a separate washer or ferrule. The thread pitch varies by nominal pipe size; a half-inch IPT fitting has 14 threads per inch, while a two-inch has 11 threads per inch. Because the thread tapers, hand-tight assembly is possible, though a wrench is always used in practice.

Leakage occurs when fittings are under-tightened, over-tightened and cracked, or when threaded connections are reused without fresh sealant. Applying thread sealant tape or pipe dope to the male threads reduces the risk of weeping at low pressures; for steam, water, or gas lines, this is mandatory. Corrosion of the threaded surfaces can make disassembly difficult and cause stripping when force is applied.

IPT is ubiquitous in plumbing, pneumatic systems, and hydraulic installations where iron or steel pipe is employed. However, it should not be confused with straight pipe thread (used in some instrumentation and hydraulic work) or with metric thread forms. When joining IPT to copper tubing or plastic fittings, transition fittings with IPT male on one end and a different connection on the other are required. Mismatches between thread forms are a common source of leaks and damage.