National Standard Taper Pipe Threads Size NPT Chart - Includes both External and internal Pipe Thread Data. The taper rate for all NPT threads is 1⁄16 (3⁄4 inch per foot) 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 1° 47'′ 24"″ (1.7899°). 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.
Nominal Pipe Size 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.