Many cake recipes and store-bought mixes only give instructions for round cakes and cupcakes, but round pans and square pans that have the same width do not hold the same volumes of cake batter. As a result, you cannot bake an 8-inch square cake for the same time you would bake an 8-inch round cake. The key to setting the correct cooking time for an 8-inch square cake is knowing the pan’s capacity.
Cake Pan Capacity
An 8-inch square pan has a capacity of 8 3/4 cups. In contrast, an 8-inch round pan has a capacity of only 7 cups. A 9-inch round pan has an 8 2/3-cup capacity, which is very close to the volume of an 8-inch square pan. If your instructions have a suggested cooking time for a 9-inch round cake, you can follow that recommendation. In general, 8-inch square cakes -- and 9-inch round cakes -- cook for approximately 25 minutes, if the cooking temperature is around 325 to 350 degrees Fahrenheit. To ensure that the cake cooks for the appropriate length of time, start testing it for doneness after 15 minutes. Remove the cake from the oven when the toothpick or knife that you insert in the center only has a few moist crumbs on it when you pull it out.