National pipe thread
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National pipe thread (NPT) is a U.S. standard for tapered threads, originally used to join steel or brass pipe and fittings. The 1/8 inch, 1/4 in, 3/8 in, 1/2 in, 3/4 in, 1 in, 1 1/4 in, 1 1/2 in, and 2 in sizes are commonly used, appearing on pipe and fittings in most U.S. hardware stores. Smaller sizes than those listed are occasionally used for compressed air. Larger sizes are used less frequently because other methods of joining are more practical at 3 inch and above in most applications.
The sizes are nominal inside diameter of the pipe when threads are used on Schedule 40 steel pipe. Because of the pipe wall thickness, the actual diameter of the threads is considerably larger than the nominal size. Schedule 80 pipe has thicker walls, but the thread profile remains the same, so the inside diameter of the pipe is therefore smaller than nominal.
These threads are now used in materials other than steel and brass, including PVC and cast iron.