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 9x2-inch round pan, use an 8x2-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.