National Pipe Thread Taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings  In contrast to straight threads that are found on a bolt, a taper thread will pull tight and therefore make a fluid-tight seal. In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864.

Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam and hydraulic fluid. These threads are now used in materials other than steel and brass, including PTFE, PVC, nylon, bronze, and cast iron.

The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft or 62.5 mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr free and lubricated using a lubricating material like lubricating paste or tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.

Commonly used sizes are ¹⁄₈, ¹⁄₄, ³⁄₈, ¹⁄₂, ³⁄₄, 1, 1 ¹⁄₄, 1 ¹⁄₂, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Sizes smaller than ¹⁄₈ inch are occasionally used for compressed air, while sizes larger than 2 inches are uncommon, due to the use of alternative methods of joining that are used with these larger sizes.

NPT is defined by ANSI/ASME standard B1.20.1.

Pipe threads are different from machine screw and bolt threads. Those are designated NC (national coarse) and NF (national fine.) The biggest difference is the taper on pipe threads.

The taper rate for all NPT threads is 1 in 16 (³⁄₄ inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance. The angle between the taper and the center axis of the pipe is tan−1(1⁄32) = 1.7899° = 1° 47′ 24″.

Nominal Pipe Size (NPS) is loosely related to the inside diameter of schedule 40 pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Other schedules of pipe have different wall thickness but the OD (outer diameter) and thread profile remain the same, so the inside diameter of the pipe is therefore different from the nominal diameter.

NPTE and NPS threads have a 60° included angle and have a sellers thread form (flattened peaks and valleys).

NPTF

A semi-compatible variant called National Pipe Taper Fuel (NPTF), also called Dryseal American National Standard Taper Pipe Thread, defined by ANSI B1.20.3, is designed to provide a more leak-free seal without the use of teflon tape or other sealant compound. NPTF threads are the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.

NPT – American Standard Pipe Thread Taper