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What are BSP thread sizes?

British Standard Pipe (BSP) thread is a standard thread type used extensively across the globe for plumbing connections (but not in North America). The standard defines the number of threads per inch (pitch) as well as the angle that each thread is engineered to. BSP threads have rounded valleys and troughs with a 55 degree pitch (this is known as Whitworth type).

There are two types of BSP threads; Tapered (BSPT) and parallel (BSPP). The vast majority of BSP threads for water fittings are parallel.

BSP sizes are always in imperial measurements and some have commonly metricised names. The table below summarises BSP sizes to help with general identification. Major diameter is slightly less than the actual outside diameter of the threads. Minor diameter is the measurement of the inside of a female threaded connection, from thread to thread.

BSP Size Also Known As Actual Diameter (Imperial) Actual Diameter (Metric)
1/8"   0.35 inch 9mm
1/4"   1/2 inch 12mm
3/8"   6/10 inch 15mm
1/2" 15 BSP 3/4 inch 19mm
5/8" 18 BSP    
3/4" 20 BSP 1 inch 25mm
1" 25 BSP 1 & 1/4 inch 31mm
1 & 1/4"   1 & 2/3 inch 40mm
1 & 1/2"   1 & 3/4 inch 45mm
2"   2 & 1/4 inch 57mm
2 & 1/2"   2 & 7/8 inch 73mm
3"   3 & 1/3 inch 85mm