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.
BSP: British Standard Pipe
BSP pipe, Like American National pipe (NPT, NPSM), is designated by trade size, rather than actual diameter, which is approximately equal to the thread's Major Diameter in the table below.
There are two types of BSP threads:
- BSPT: British Standard Pipe Taper -also known as "R" or "Rc" threads
- BSPP: British Standard Pipe Parallel -also known as "G" or "Rp" threads
Both styles have the same thread angle, shape, and pitch (threads per inch). However, BSPT threads are tapered and BSPP threads are straight (parallel). BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).
Here are the actual thread dimension data for BSPP and BSPT threads. The major diameter is a bit
larger smaller than the actual OD of the pipe, and the minor diameter should be very close to what you would measure inside the female threaded end of a fitting. Note that the Gage Length dimension pertains only to the BSPT (tapered) thread.
NPTR - National Pipe Taper Railing
While NPT threads are common in the United States, BSP threads are widely used in many other countries. I have found that my Japanese-built injection mold presses have predominantly BSP fittings.