The car wash mop head unit is 9 ½” in diameter in order to cover more area per pass making the job quicker and easier. The mop head includes a soft rubber edge to prevent the mop head from cutting though the bonnet and making scuff marks if the user applies to much pressure or accidentally hits the vehicle with the car wash mop head. Simply put the bonnet on the Mop Head base and screw the pole onto the mop head base and you're ready to go.