Installs on either a 3 or 4inch round downspout.  It's pretty tight on the 3" and takes a bit of rotating to get it to seat. Take your time as you squeeze it in the hole.  The diverter and the hole in the side of the container MUST BE on the SAME horizontal plane for it to work as designed. Water follows the path of least resistance. Once the container has filled the water will continue down the downspout.