Threaded pipe joints

Entry in Joining methods · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

NPT threads (National Pipe Taper) are the tapered pipe threads used on most US threaded plumbing joints, defined in the standard ASME B1.20.1, Pipe Threads, General Purpose (Inch).^[2] Because the threads taper, a male and female end wedge tighter as they are turned together, and the seal forms on the threads themselves.^[1]

Threaded joints join galvanized steel pipe, brass nipples and fittings, valves, water heater and appliance connections, and the threaded adapters used to move between materials. The same thread form is used on black steel gas pipe; gas piping is installed and altered only by qualified, licensed people under the local fuel gas code, and nothing here is a guide to that work.

1 Thread form

An NPT thread has a 60° thread angle, flat-truncated crests and roots, and a taper angle of 1° 47'; its pitch is stated in threads per inch.^[1] ASME B1.20.1 also defines straight (non-tapered) pipe threads, such as NPSC for the inside of couplings and NPSM for fixture and mechanical joints; these are not tapered.^[2]

The outside diameters show why a "1/2 in" threaded pipe measures well over half an inch; see nominal pipe size. NPT and the British BSPT thread are close enough in 1/2 and 3/4 in that they can be hard to tell apart without gauging.^[1]

2 Sealing

Tapered pipe threads always need a sealant to seal and to reduce galling of the threads; PTFE tape and pipe thread sealants serve as both lubricant and sealer.^[1] There is no standard torque or number of turns for an NPT joint.^[1] State codes repeat this: Massachusetts requires all threaded pipe joints to follow American National Taper Pipe Thread and allows only thread tape, lubricant seal or other accepted material recommended by the maker.^[4]

3 Plastic threads

A tapered thread acts as a wedge, and over-tightening can split a plastic female fitting; smaller sizes split more easily.^[3] One plastic pipe maker recommends against threading a metal male end into a plastic female fitting, limits plastic threaded connections to two turns past finger tight with a strap wrench, uses PTFE tape only on 1 in and smaller, and derates plastic threaded fittings an additional 50% below the pipe's pressure rating.^[3] The same manual calls for thread sealants that meet IAPMO PS 36 and are confirmed compatible with ABS, CPVC and PVC, and warns against sealant on trap slip-nut connections.^[3]

Where plastic water pipe meets a threaded metal part, makers offer brass-threaded transition fittings; a copper-to-galvanized connection calls for a dielectric union or other approved fitting.