<P> Commonly used sizes are ​ ⁄, ​ ⁄, ​ ⁄, ​ ⁄, ​ ⁄, 1, ​ 1 ⁄, ​ 1 ⁄, and 2 inch, appearing on pipes and fittings by most U.S. suppliers . Sizes smaller than ​ ⁄ 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 . </P> <P> NPT is defined by ANSI / ASME standard B1. 20.1 . </P> <P> The taper rate for all NPT threads is 1 inch of diameter in 16 inches of length (⁄ inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance of thread . The angle between the taper and the center axis of the pipe is tan (​ ⁄) = 1.7899 ° = 1 ° 47 ′ 24". </P> <P> The standard for Nominal Pipe Size (often abbreviated NPS, which should not be confused with the symbol NPS for the straight thread form standard) is loosely related to the inside diameter of Schedule 40 series of sizes . Because of the pipe wall thickness of Schedule pipe, the actual diameter of the NPT threads is larger than the Nominal Pipe Size outside diameter, and considerably so for small sizes . Pipe of a given size in a different Schedule than Schedule 40 provides a different wall thickness, while maintaining the same outside diameter and thread profile as Schedule 40 . Thus the inside diameter of a given size of Schedule pipe differs from the Nominal Pipe Size, while outside diameters for a given nominal Schedule size are the same between Schedules . </P>

Basic dimensions for npt american national taper pipe threads