BSP vs NPT

You may have noticed that there a range of fittings used for water filtration systems which use the imperial inch scale and others which use the metric scale.

When comparing NPT to BSP there are some fittings which are the same and others which are different and incompatible.

NPT stands for National Pipe Taper and common in the USA and BSP stands for British Standard Pipe and are used widely around the world.

NPT and BSP threads are generally not compatible due to the differences in their thread forms. NPT 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.

You can use 1/2" and 3/4" NPT and BSP fittings together, but all others will be incompatible and will therefore leak.

 
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 1/211
1 1/4"11 1/211
1 1/2"11 1/211
2"11 1/211
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 ---