Where is the "one-eighth" in 1/8-27 NPT? I know it has 27 threads per inch. I see references that 1/8" is the nominal pipe size, does that mean the hole inside the thread is 1/8" ID? Or something else?


2 Answers 2


Threaded pipe sizes seem weird if you think they are like threaded fastener sizes, because they are not. They use a tapered thread, and the outside diameter of the pipe is not the "size" of the pipe.

They also are weird, because the fixed size of pipe is the outside diameter of the pipe, so the fittings will all work with any pipe of that size, but the nominal pipe size is the inside diameter and the actual inside diameter varies with the wall thickness or "schedule" of the pipe. So a 1/8" schedule 5 pipe has a considerably larger hole (0.405") than a 1/8" schedule 80 pipe (0.215".) And neither of those holes are 0.125" - just put it down to "some things are weird," and move on because logical consistency is hard to come by, other than "generally the hole in the pipe is somewhat larger than the nominal size" with a side order of "except when it's not."

1-1/2" Schedule 80 is the weirdo with an actual ID of 1.500" - I presume all the other pipes avoid them at parties.

  • 2
    it's a weird world where the weirdo is not weird
    – P2000
    Apr 19, 2022 at 18:44

1/8" was the nominal inside diameter of the pipe, back when they first started making standardized pipe. That is a matter of the limits of the metallurgy and fabrication methods at the time.

As metallurgy improved, they were able to achieve the same strength with thinner walls. Now what?

They still need to fit legacy fittings. But those attach to the outside diameter. To avoid having 100 different sizes of fitting, they decided to keep the outside diameter the same and modify the inside diameter to suit the material thickness.

So, much like modern railroad track width being defined by Roman horse carts, we owe it all to history!

  • Indeed at en.wikipedia.org/wiki/Nominal_Pipe_Size, and plausible, but haven't found a primary reference for this. You?
    – P2000
    Apr 19, 2022 at 19:01
  • Roman horse/mule carts were also the reason for the size of the shuttle's booster rockets(might be an urban legend)
    – crip659
    Apr 19, 2022 at 23:58
  • 2
    @crip659 because booster segments are shipped by rail, because they had to be built in Utah for political reasons. Apr 20, 2022 at 0:33

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