British Standard Pipe Thread is a family of standard screw thread types that has been adopted internationally for interconnecting and sealing pipe ends by mating an external (male) with an internal (female) thread.
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.
A 1/2 Inch backnut
As the name of the sizing does not match the actual diameter, there can be some confusion over sizing. Please see the table below and note the outside diameter mm being the actual mm.
Typical uses and sizes are :
1/4 Inch - Compressed air connections
3/8 Inch - Oil line connections
1/2 Inch - Basin and sink taps, radiator inlets, old outside tap outlets
5/8 Inch - Old outside tap outlets (very rare)
3/4 Inch - Bath taps, washing machine inlet hose connections, modern outside tap outlets
1 Inch Domestic heating boilers
1.25 Inch - Wash hand basin wastes (bathroom sink)
1.5 Inch - Kitchen sink wastes (kitchen sink), bath wastes, WC syphon outlets
2 Inch - Modern toilet flushvalve outlets