Pipe sizes do not refer to any physical dimensions. The outside diameter of a pipe or fitting must be measured and compared to a table for size identification. A 3/4" NPT pipe thread has an outside diameter - OD - of 1.050 inches.
Each thread size has a defined number of threads per inch - TPI, or pitch. The 3/4" NPT pipe thread has 14 threads per inch. Both the TPI and OD of the thread are required for positive identification of thread size because more than one size have the same TPI.
Pipe Size (inches) / Threads per Inch TPI - pitch / Nominal Outside Pipe / Diameter OD (inches) / Tap Drill (inches)
1/16" 27 0.313
1/8" 27 0.405 R
1/4" 18 0.540 7/16
3/8" 18 0.675 37/64
1/2" 14 0.840 23/32
3/4" 14 1.050 59/64