ANSI/ASME standard B1.20.1 covers threads of 60-degree form with flat crests and roots in sizes from ¹??? inch to 24 inch Nominal Pipe Size (this standard also covers various 'straight' threads, see [1]). The taper rate for all NPT threads is ¹??? (¾ inch per foot) measured by the change of diameter (of the pipe thread) over distance. The taper divided by a center line yields an angle 1° 47' 24" or 1.7899° as measured from the center axis. Commonly-used sizes are ⅛ , ¼, ⅜, ½, ¾, 1, 1 ¼, 1 ½, and 2 inch, appearing on pipe and fittings by most U.S. suppliers. 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 inches and above in most applications.

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.

Pipe thread standard from Wikipedia, the free encyclopedia

How to measure National Pipe Thread fittings: The NPTF (National Pipe Thread Fitting) male has tapered threads and a 30 degree inverted seat. The NPTF female has tapered threads and no seat. The seal takes place by deformation of the threads. The number of threads per inch varies by size and the thread angle is 60 degrees.      NPTF Solid Male  NPTF Solid Female 1) With a caliper, measure the thread diameter at the widest point across the threads (See image below).   Outside diameter (O.D.) of male threads. Inside diameter (I.D.) of female threads.       2) Compare measurements to chart below. For example, a 3/8” male pipe thread is actually 43/64” (slightly less than 11/16”) in outside diameter. Nominal Size    Female Thread I.D. (In.)  Male Thread O.D. (In.) No. of Threads Per Inch 1/8 23/64 13/32 27 1/4 15/32 35/64 18 3/8 19/32 43/64 18 1/2 3/4 27/32 14 3/4 61/64 1 1/16 14 1 1 13/64  1 5/16 11 1/2

The how to measure information courtesy of