Australian BSP thread size names can be confusing. The mm and inch naming system are only nominal conventions, they are not the physical measurements of the thread size. E.g. a common 15mm (1/2") fitting will have an actual external male thread diameter or an internal female thread diameter measurement of 20.995mm (0.825"); 20mm (3/4") fitting likewise will measure 26.441mm (1.041")
Please refer to our BSP sizing chart provided in the last image in our gallery. Feel free to asks us any questions.