Finding the Correct Pipe Thread Size
National Pipe Threads (NPT)
Pipe sizes do refer to physical dimensions, but the plumbing industry
standard for the sizes of pipe is not always as straight-forward as
measuring the pipe. Measuring the pipe will oftentimes mislead you into choosing the wrong pipe, so do take care.
The "pipe thread size" as shown in column 3 of the table below is
measured and based on the inside of the pipe. But, to actually determine
the size of a pipe, the outside diameter of each pipe or fitting must
be measured and compared to the table for size identification. For
example, a 3/4" NPT pipe thread has an outside diameter of 1.050 inches.
Each thread size has a defined number of threads per inch (TPI). The
3/4" NPT pipe thread has 14 threads per inch. Both the TPI (threads per
inch) and OD (outside diameter) of the thread are required for positive
identification of thread size because several sizes have the same TPI.