Swap pan sizes without ruining your recipe.
The key is comparing surface areas. A 9-inch round pan has 63.6 sq inches; an 8-inch square has 64 sq inches — nearly identical, so recipes swap directly. A 9x13 rectangle (117 sq in) holds about 1.84× the batter of a 9-inch round. To convert: multiply all ingredients by (new area / original area). Common equivalents: two 9-inch rounds ≈ one 9x13 rectangle ≈ three 8-inch rounds. Fill pans ½ to ⅔ full for proper rising. When scaling up, increase baking time 5-10 minutes; when scaling down, reduce by 5-10 minutes. Temperature stays the same. For deeper cakes, add a collar of parchment paper above the rim.