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Hose is like flexible tubing, but it should generally be considered as PIPE.

Hose is sized by its Inside Diameter. (A notable exception to this is the case of FIRE HOSE, which is often sized by its O.D. due to its associated fitments, many of which use the O.D. as their determinant diameter.)

Hose is most typically (and traditionally) made of rubber, though modern technology has brought us myriad new materials suitable for various working fluids and physical working criteria.

American National pipe (NPT, NPS), Like British Standard Pipe (BSP), is designated by trade size, rather than actual diameter, as shown in the table below.

There are two basic types of National pipe threads:

  • NPT: National Pipe Taper
  • NPS: National Pipe Straight

NPT threads are also sometimes referred to as

  • MIP (Male Iron Pipe)
  • FIP (Female Iron Pipe)
  • IPT (Iron Pipe Thread)
  • FPT (Female Pipe Thread)
  • MPT (Male Pipe Thread)

Note that these references are somewhat casual, and might possibly be used in reference to NPS instead of NPT.

Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch).  However, NPT threads are tapered and NPS threads are straight (parallel).  Both threads have a 60° included angle and have flat peaks and valleys (this is a Sellers thread form).

If you've worked with pipe much at all, you've probably noticed that the size of the pipe isn't really what size the pipe is.  Unlike tubing, which is generally specified by its OD, or hose, which is generally specified by its ID, pipe is specified by something else... its Trade Size.  So when you say "3/4 pipe," you're actually saying "pipe whose OD is a little more than an inch, and whose ID is about 53/64."  -that is, if you are talking about schedule 40 pipe, which is generally what is used for most plumbing applications.

Pipe dimensions are specified by trade size and schedule, according to the following table.  Note that while British Standard Pipe dimensions are similar, they are not equivalent to the American Standard Pipe Sizes.  See NPT vs. BSP Pipe for thread comparisons.

While NPT/NPS threads are common in the United States and Canada, BSPT/BSPP (collectively, BSP) threads are widely used in many other countries. I have found that my Japanese-built injection mold presses have predominantly BSP fittings. 

  • BSPT -British Standard Pipe Taper
  • BSPP -British Standard Pipe Parallel
  • NPT -National Pipe Taper
  • NPS -National Pipe Straight

While the actual specified outside diameters of American National Pipe differ slightly from those of British Standard Pipe, either thread may reliably be cut onto a pipe of its respective trade size.  BSPT and BSPP threads are analogous to NPT  and NPS threads, respectively.

WARNING:  Never, never try to mate a BSP fitting with an NPT or NPS fitting if the pressure holding capability is at all critical.

NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).

NPT and BSP thread pitches (threads per inch, TPI) are listed below. To determine pitch, use a thread gauge or count the number of threads that fall into a 1" span.  Note that, strictly speaking, when we use threads per inch, we are actually specifying the inverse of the pitch, pitch being in units of [length] / [peak to peak].  Metric threads are usually specified in actual pitch, e.g., 1.5mm, 2.0mm, etc.  This is the actual length of each thread, peak to peak.  Although the term "pitch" is universally used, albeit loosely, to describe threads per inch, the actual pitch of a 1/4BSP fitting is really 1/19 inch, or 0.0526 inches.

 

 

 

Pipe Size

Pitch  (Threads/Inch)

NPT/NPS

BSP

 1/16"

27

---

 1/8"

27

28

 1/4"

18

19

 3/8"

18

19

 1/2"

14

14

 5/8"

---

14

 

Pipe Size

Pitch  (Threads/Inch)

NPT/NPS

BSP

 3/4"

14

14

1"

11 1/2

11

1 1/4"

11 1/2

11

1 1/2"

11 1/2

11

2"

11 1/2

11

2 1/2"

8

11

 

Pipe Size

Pitch  (Threads/Inch)

NPT/NPS

BSP

3"

8

11

3 1/2"

8

11

4"

8

11

5"

8

11

6"

8

11

8"

8

---

 

 

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