Do you understand how to properly measure pipe threads? Â
Pipe sizes do refer to physical sizes but it's not as straight-forward as some may think. Â
Threaded pipes and fittings are standardized in the U.S. and are denoted as NPT or National Pipe Thread Taper. Â
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The nominal "Pipe Thread Size", shown in column 3, is based on the inside diameter of the pipe. Â 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.
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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".
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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.
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