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NPT Pipe Threads - Coyote Gear

When does 1" never measure 1"? Answer: when we are working with pipe, pipe fittings and pipe threads. When I first started working with piping products, many decades ago, this subject made my head spin.

  • NPT is an acronym for National Pipe Thread. NPT threads are tapered, which means that the thread diameter becomes larger as the threads progress. The taper is intended to improve sealing of the joint because it wedges the threads together.
  • NPT sizes are often misunderstood. NPT pipe thread sizes do not correspond to actual inch measurements. For example a 1” NPT Pipe Thread has a diameter of approximately 1.315”. It's approximate because the thread is tapered so the actual diameter changes depending on at what point you measure the threads.

NPT Taper

  • Another misunderstanding is that 1” is the inside diameter of a 1” pipe-that's not true either because depending on the schedule of the pipe or the PSI rating of the fitting the ID from one type of part to another is not the same.
NPT Size 1/16 1/8 1/4 3/8 1/2 3/4 1 1-1/4 1-1/2 2 2-1/2 3 4 6 8
Diameter 0.3125" 0.405" 0.540" 0.675" 0.840" 1.050" 1.315" 1.660" 1.90" 2.375" 2.875" 3.5" 4.5" 6.625 8.625"
TPI 27 27 18 18 14 14 11.5 11.5 11.5 11.5 8 8 8 8 8

 

I hope this information helps people who are new to piping products get the size parts they need.

There is more piping information below in our "Resources" section.

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