These are external compression fittings, giving no obstructions inside the pipe for maximum flows. Suitable for MDID pipe and most LD alkathene pipe
Measured by the internal diameter in pipe using traditional imperial sizes. ( Inch, 3/4 etc.) 20mm, 25mm etc
Very efficient gripping rings on the pipe exterior walls gives good internal flows but pipe external diameter has to be correct for gripping successfully.
note -50mm RX compression fitting do not fit all LD 50mm- just MDID pipe.
note -40mm pipe and fittings are only made for approx. 38mm actually