Switching pan shapes changes how thick your batter sits, which changes how long it needs to bake. Enter your original and new pan to scale the recipe and estimate a new bake time.
Cake and bar recipes are calibrated to fill a pan to a certain depth. Converting between pan sizes and shapes means calculating each pan's surface area, then scaling the recipe by the ratio of the new area to the old one. Because a thinner layer of batter bakes faster than a thicker one, bake time is adjusted in the opposite direction of the area change.
Source: Pan area-based conversion is the standard approach recommended by baking references including King Arthur Baking's pan substitution guidance.
Calculate the area of your round pan (π × radius²) and the area of the rectangular pan (length × width), then scale every ingredient by the ratio of the new area to the old one.
Batter depth changes even when the recipe amount doesn't. A larger pan spreads the same batter thinner, so it bakes faster; a smaller, deeper pan needs more time for the center to cook through.
Area is the most useful measure for typical cake/bar pans of similar depth, since batter is poured to a similar fill level. For very differently shaped pans (like a bundt vs. a sheet pan), also compare volume capacity and use your judgment on fill depth.
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