MEASUREMENTNo exact measurement from the actual fitting determines the size (in millimeters). The size is the inner diameter of the tube (or hose) that will be installed onto the fitting. Shown below is a 19mm V2 quick connect fitting. NOTE: it is not necessarily the ID or the OD of the fitting. The size will be in between these 2 values.IMPORTANT: Always lubricate the mating O-rings with a light amount of oil prior to installing.