Baking is a complicated art. One false move could ruin your whole pie, cake or batch of muffins. If you don't have the exact size baking pan a recipe calls for, use the two-thirds rule as a guideline when you swap sizes.
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This simple rule of thumb comes via Epicurious. They write:
Generally you only want to fill the cake pan half to two-thirds full so the batter doesn't pour over the sides. (Unless, of course, the recipe specifically says otherwise.) For heavier batters, such as banana breads and pumpkin bread, two-thirds is fine, says Medrich. But light and spongier cakes will rise more, so only fill these pans half-way.
Two-thirds is the general rule, but if you're in doubt, though, they actually recommend erring on the side of underfilling: Stick to half full if you're trying a recipe for the first time using a different size pan or the recipe doesn't tell you how much to fill. For more baking tips, head to the full post at the link below.
6 Rules for Swapping Baking Pans [Epicurious]