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 inch BSP thread actually measures more like 1 inch 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 inch BSP - Compressed air connections.

3/8 inch BSP - Oil line connections.

1/2 inch BSP - Bathroom basin and kitchen sink taps, radiator inlets, old outside tap outlets.

5/8 inch BSP - Old outside tap outlets.

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

1 inch BSP - Domestic heating boilers and hot water cylinders.

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

1-1/2 inch BSP - Kitchen sink wastes, bath wastes, toilet syphon outlets.

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

The table below shows dimensions for male threads :

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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