The national standard for NPT (National Pipe Taper) taper pipe threads is ASME B1.20.1, the current edition has a date of 2013.
This standard contains product detail values and their calculation formulas as well as conformance requirements. It also contains gage design details used to evaluate only some of the conformance requirements. Yes, you read that correctly, used to evaluate only some of the conformance requirements.
Taper pipe threads are made up of, diameters, flats, angles, and a taper. The diameters are located along the thread