National Pipe Taper (NPT) are a standardized fitting that is commonly used for joining pipe. Due to the tapered threads (wider diameter at the base of the threads, similar to a trapezoid) the male fitting will tighten the further it is threaded into the female. NPT follows the technical standards provided by the American National Standard Pipe Thread standards, often called national pipe thread, which is often where the confusion with NPT arises, as NPT is specifically an acronym that refers to tapered pipe.

National Pipe Straight (NPS) are straight thread that is sized to be compatible with common water heater elements and should be compatible with most NPS straight threads.

Determining NPT/NPS Size:
Sizing NPT/NPS is based on the nominal pipe diameter, and not the outside diameter (OD) of the threads. For example: A 1/2 " NPT male has an outside thread diameter of approximately 3/4 ", but as stated the correct NPT size is 1/2 ", including the counterpart female.

Notice: While NPT and NPS threads may be joined together, their threads may not form a proper seal, be loose fitting, or otherwise not an optimal connection.