FPT or MPT thread size identification by actual dimensions of the fitting or valve thread.

FPT or MPT thread size identification by actual dimensions of the fitting or valve thread.

Piping sizes are nominal inside diameters of IPS (Iron Pipe Size) pipes.

It is quite common for someone to measure a thread on an existing part that needs replacement and to make an error on what size of replacement part they require.

For example, for a 1” MPT thread that measures close to 1-1/4” at the small end of the taper (1.256”), it is assumed in error that this is a 1-1/4” MPT connection, when in fact it is a 1” MPT pipe thread (1” pipe measures 1.315” O.D.).

The diagrams below provide the actual dimensions for both female (FPT) and male (MPT) threads to assist in thread identification.

Additional Resources:

https://support.boshart.com/what-the-difference-between-iron-pipe-size-/-nominal-pipe-size-vs.-copper-tube-size

https://support.boshart.com/what-are-the-dimesnions-of-schedule-40-steel-pipe

https://support.boshart.com/making-npt-connections

https://support.boshart.com/what-is-the-proper-thread-engagement-for-npt-national-tapered-pipe-threads-to-asme-b.1.20.1-standard

https://support.boshart.com/how-to-apply-ptfe-thread-seal-tape

https://support.boshart.com/making-a-leak-proof-stainless-steel-threaded-connection

https://support.boshart.com/can-the-gray-tt260-ss-p.t.f.e.-tape-be-used-on-other-materials-other-than-stainless-steel

https://support.boshart.com/can-the-gray-tt260-ss-p.t.f.e.-tape-be-used-on-other-materials-other-than-stainless-steel