British Standard Pipe Thread (BSPT)

The threaded part(s) of plumbing fittings are usually made to a British Standard Pipe Thread size (BSP).

BSP sizes don't match up with what you measure on the scale of a ruler, which can obviously cause some confusion. For example a 1/2 inch BSP thread (see above) actually measures more like 3/4 inch with a ruler.

The size was originally based on the inside diameter, measured in inches, of a steel pipe for which the thread was intended. This is the source of the confusion, as people incorrectly think the size refers to the outside diameter of the male thread.

Two types of threads are used:

Parallel (straight) threads, British Standard Pipe Parallel thread (BSPP). These are sometimes denoted by the letter G, for example G1/2" means a parallel 1/2 inch BSP thread.

Taper threads, British Standard Pipe Taper thread (BSPT). These are sometimes denoted by the letter R, for example R1/2" means a taper 1/2 inch BSP thread.

Just to confuse things further, often the T in BSPT refers to Thread and doesn't indicate either Taper or Parallel.

Typical sizes and uses are :

1/4" BSP - Compressed air connectors.

3/8" BSP - Central heating oil line connectors, some European toilet cistern fill valves (not common in UK).

1/2" BSP - Bathroom basin taps and kitchen sink taps, radiator connections, shower hoses, old outside tap outlets, toilet cistern fill / ball / float valves.

5/8" BSP - Old outside tap outlets (we have an adapter for this size).

3/4" BSP - Bath taps, washing machine inlet hose connections, modern outside tap outlets.

7/8" BSP - Outlet thread on 3/4 inch outside tap (we have a 7/8 inch BSP brass garden hose tap connector).

1" BSP - Domestic heating boilers and hot water cylinders, urinal flush valves.

1-1/4" BSP - Wash hand basin (bathroom sink) wastes.

1-1/2" BSP - Kitchen sink wastes, bath wastes, toilet syphon outlets, central heating pumps.

2-1/4" BSP BSP - Immersion water heater elements.

The BSP to mm conversion chart below shows metric and imperial dimensions for male threads and the number of threads per inch (TPI) :

The information here is provided for guidance only. The data shown represents extracts interpreted by us of the referenced standards. If more clarification and accuracy is required, you should refer to the relevant standard.