BSP Sizing VS Physical Measurement Chart

15mm, 1/2", DN15, BSP? I am sure you have seen these terms thrown around on plumbing website, at your local plumbing supply shops, or heard from plumbers. Then you take a ruler out to measure the thread old fashion way and it is not any of the sizes you saw advertised. What is going on and confused?
Australian Plumbing industry uses BSP (British Standard Pitch) sizing when defining the thread sizes of valves and fittings. The BSP Metric and Imperial sizing is just nominal and used as a naming convention. They are not the actual physical measurements of the thread size.
For example, the most common thread size for most common fittings and valves are known as BSP 1/2", 15mm or DN15. These 3 terms here all refer to the same thread. They are freely exchangeable in plumbing catalogs or just chatting with a plumber or over a plumbing supply shop counter. i.e. you will get exactly the same fitting/valve, if you tell the plumber that you want a 1/2" or DN15 or 15mm thread size. When measuring the external thread diameter (if a male thread) or the internal thread diameter (if a female thread), you will get a physical measurement of 20.995mm. Now you can see how first time DIY person will mistaken their 15mm garden hose tap as 20mm item when shopping online, because they feel 20.995mm is closest to 20mm BSP sizing by the name.
Likewise, the same appies to 3/4", 20mm, DN20 and a physical thread measurement of 26.441mm as a group; or 1", 25mm, DN25 and a physical thread measurement of 33.249mm as a group; etc. 
The table provided here best explains the above. 
Just remember BSP 1/2" or 15mm is most common for residential applications. If you are not sure what size you need, just email All Plumbing Online a photo or do a physical measurement of your thread. 

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