NPT threads are an American standard 60 degree tapered pipe thread with detailed and specific sizing criteria. The following is a specific description of NPT thread size:

First, the basic parameters of NPT thread

Thread type: tapered pipe thread, with the basic profile of 60 degree truncated triangle, similar to the imperial thread but with taper.
Rotation: right-hand rotation by default, left-hand rotation should be specially marked (e.g. -LH).
Unit: Size is usually in inches, pitch is expressed in TPI (teeth per inch) or distance (e.g. inches).

Second, the specific size of NPT thread is as follows:

ANSI B1.20.1-NPT-American National Standard Taper Pipe Threads¹)

Cited from: https://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html

Third, the sealing of NPT threads:

The tapered design of NPT threads allows them to form a seal when twisted, as the sides of the external and internal threads compress against each other.

IV: Application of NPT threads:

NPT threads are widely used for connection of fuel pipes, oil pipes, water pipes, gas pipes, etc. on machines.

V: NPT thread precautions:

In actual application, suitable NPT thread specifications should be selected according to specific needs and standards.
The processing and installation of threads should follow the relevant standards and specifications to ensure the reliability and sealing of the connection.

In summary, NPT thread size standards are detailed and specific, covering parameters from nominal diameter to taper and other aspects. When selecting and using NPT threads, relevant standards and specifications should be carefully checked to ensure the quality and performance of the connection.