It takes two 18-ounce boxes of cake mix to make a half sheet cake, measuring approximately 12-by-18 inches. A half sheet cake provides 36 to 108 servings, depending on how it is sliced. Commercially prepared half sheet cakes are often made by assembling two 9-by-13-inch cakes on a cake board.
Before baking a half sheet cake, butter and flour the cake pans, and add a layer of parchment paper to the bottom of each pan. This makes it easier to remove the cakes without them breaking once they have cooled. Never attempt to take a warm cake out of the pan. To assemble a half sheet cake from two smaller cakes, place the cakes side by side, with the long sides touching, on a cake board.