Nominal Pipe sizes do not match any of its actual physical dimensions
The OD of pipe / fitting must be measured & compared to the table for  identification
For example, a 3/4" NPT pipe thread has an outside diameter of 1.050 inches and 14 threads per inch.  Each thread size has a defined number of threads per inch (TPI), but several sizes have the same TPIbecause of this, both the TPI and OD of the thread are required for positive identification of thread size.

 Male threads:   Measure the outside diameter of the large portion of the thread at "A"; Find figure nearest this dimension in column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size. Female Threads:  Measure top diameter of thread at "B"; Find figure nearest this dimension in  column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size.
Column 1 Column 2 Column 3 Column 4 Column 5
OD
Fraction  Inch
(only for quick reference )
Actual

Decimal Inch

Pipe Size

Nominal

TPI

Normal Engagement
for a tight joint (dimension "C")
5/16" 0.3125" 1/16" 27 0.2611
13/32" 0.405" 1/8" 27 0.2639
35/64" 0.540" 1/4" 18 0.4018
43/64" 0.675" 3/8" 18 0.4078
27/32" 0.840" 1/2" 14 0.5337
1-3/64" 1.050" 3/4" 14 0.5457
1-5/16" 1.315" 1" 11-1/2 0.6828
1-21/32" 1.660" 1-1/4" 11-1/2 0.7068
1-29/32" 1.900" 1-1/2" 11-1/2 0.7235
2-3/8" 2.375" 2" 11-1/2 0.7565
2-7/8" 2.875" 2-1/2" 8 1.1375
3-1/2" 3.500" 3" 8 1.2000
4" 4.000" 3-1/2" 8 1.2500
4-1/2 4.500" 4" 8 1.3000

Two
common pipe thread sizes exist, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM).
The tapered threads are for joining and sealing, the straight threads are only for joining.
Dry-seal thread (NPTF) allows for joining without sealants.
Female NPT threads may be designated as "FPT"
Male NPT threads may be designated as "MPT."
The pipe size is often written such as this 1/8"-27, with a hyphen between size & thread per inch, and pronounced as "one eighth twenty-seven"