I ordered this one inch BSP (British Standard Pipe) adapter after using a steel rule to measure the outside thread diameter of my garden tap as one inch. HOWEVER, the BSP size DOES NOT relate to the diameter of the thread you measure with your ruler! This adapter actually measures 1.25 inches across the inside thread so it's too big for the tap. I actually require a three quarter inch BSP adapter for a one inch outside thread on the tap! I had to find all this out from Google after ordering two different adapters that don't fit! Look for yourself. I don't understand it either!