Do you understand how to properly measure pipe threads?
Pipe sizes do refer to physical sizes but it's not as straight-forward as some may think.
Threaded pipes and fittings are standardized in the U.S. and are denoted as NPT or National Pipe Thread Taper.
The nominal "Pipe Thread Size", shown in column 3, is based on the inside diameter of the pipe. But, to actually determine the nominal "Pipe Thread Size", the outside diameter of the pipe or fitting must be measured and compared to column 1 or column 2 in the table for size identification.
Male Threads: Male (external threads) are usually referred to by with one of the following abbreviations: MPT, MNPT or NPT(M). Measure the outside diameter of the large portion of the thread at "A". Find the dimension nearest this dimension in Column 1 or 2. The dimension in Column 3 will be your nominal "Pipe Thread Size".
Females Threads: Female (internal threads) are usually referred to by with one of the following abbreviations: FPT, FNPT or NPT(F). Measure the top diameter of thread at "B"; Find the dimension nearest this dimension in Column 1 or 2. The dimension in Column 3 will be your nominal pipe thread size.