The sandbox should be filled with sand to a depth of around 4" or 10cm. At 40"x30", this totals 2.77 cubic feet or 75dm3 or 75l of sand, weighing approximately 198lb or 98kg. We recommend Sandtastik White Play Sand, for between $15 and $25 per 25lb, or $120-$200 total. Sandtastik sand has excellent projection properties, but a shallow angle of repose when dry. We recommend keeping the sand slightly moist to make it moldable. Adding 1 cup or 0.25l of water to 198lb or 98kg of sand and mixing thoroughly is sufficient.