Description from Ikea

Mount the short side on a 7⅞” deep shelf and the long side on a 11⅞” deep shelf. Screws are not included. Different wall materials require different types of fasteners. Use fasteners suitable for the walls in your home. If you are uncertain about what type of mounting devices to use, please contact your local hardware store. If using only 2 brackets on a 47¼” shelf, the distance between the brackets should be 27½-31½”.