Please choose from the size ranges offered. You want a water tray that is a bit larger in both length and width than your planter box. Then in the comment box under Advanced Options, please let us know the exact size of your planter. The dimension we will need to make a perfectly sized water tray for you is the length and width of the box as it sits on the ground (the footprint). We recommend you add at least one inch in both length and width to this measurement so you have some space between your planter and the interior wall of the water tray. This will allow you to view the water level at any one time.