what’s difference between different thread size

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).