Calculating BSP Thread Sizes

A frequent problem that we find many of our customers have is the identification and measurement of pipe threads.

BSP known as British Standard Pipe can be extremely confusing, especially when measuring existing fittings in need of replacement.

Many people think that BSP Thread size is the dimension of the outer diameter of the male threads, however this is not the case. To determine the thread size, you will need to subtract ¼” (0.25”) from the outer measurement to specify the correct fitting. Using the table below we can see that a thread measuring 1/2" is actually a 1/4" BSP thread. Similarly, a thread that measures 1" is actually a 3/4" BSP thread.

Thread Types:

BSPP Parallel (Straight) Threads - these are parallel to the centreline and normally seal on with a washer or O-ring.

BSPT Taper Threads - these are at an angle in comparison to the centreline and sealed by two pairing threads.

Thread per Inch

To also make sure the fitting is BSP, you can count the number of threads over ¼” length then multiply by 4 to get the amount of threads per inch.

For example, if there were 7 thread crests in ¼” you would multiply 7 (thread crests) x 4 (conversion to inch) = 28 (Threads per Inch).

Alternatively you can use a screw pitch gauge to check too.

How to measure BSP Thread Sizes

•  To begin you will need either a ruler or vernier guage to measure either across the thread O/D if male and inside threads I/D if female.

•  Take your ruler and place it across to measure the diameter threads. (Take note this is not the BSP thread size)

•  When you have your measurement, for example approx 1” this will not be the BSP size. Checking the table below, you can see that you will actually need 3/4” BSP Thread Size.