Say you want to make a cake, but you don’t have the exact pan called for in the recipe–what do you do? You could run to the store for the right pan, or you could save yourself some time and money by baking the cake in a pan that you already have.
A round pan holds about 25% less than a square pan of the same width, so to convert from a round cake to a square cake, just go down one pan size. For example, if your recipe calls for a 9×2-inch round pan, use an 8×2-inch square pan.
If converting to other types of pans-perhaps a heart shape for Valentine’s Day–fill the pan called for to the brim with water, then pour the water into a shape you like to see if it has the same volume.