British Standard Pipe Thread

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

BSP sizes don’t match up with what you would measure on the scale of a ruler, which can obviously cause some confusion. For example a 3/4" BSP thread actually measures more like 1" with a ruler.

The size was originally based on the inner diameter measured in inches of a steel tube for which the thread was intended. This often causes confusion, as people 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).

Taper threads, British Standard Pipe Taper thread (BSPT). Just to confuse things, 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 connections.

3/8" BSP - Oil line connections, some European toilet cistern fill valves (not common in UK).

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

5/8" BSP - Old outside tap outlets.

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

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

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

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

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

The table below shows dimensions for male threads :

British Standard Pipe Thread Sizes

BSP Size

Outside Diameter

Threads per inch

1/16"

7.7mm

0.304"

28

1/8"

9.7mm

0.383"

28

1/4"

13.16mm

0.518"

19

3/8"

16.66mm

0.656"

19

1/2"

20.99mm

0.825"

14

5/8"

22.99mm

0.902"

14

3/4"

26.44mm

1.041"

14

1"

33.25mm

1.309"

11

1-1/4"

41.91mm

1.650"

11

1-1/2"

47.80mm

1.882"

11

2"

59.61mm

2.347"

11

2-1/2"

75.18mm

2.96"

11

3"

87.88mm

3.46"

11

4"

113.03mm

4.45"

11

5"

138.43mm

5.45"

11

6"

163.83mm

6.45"

11

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