Standard pan area cheat sheet

13x9 = 117 square inches

9x9 = 81 square inches

8x8 = 64 square inches

Half sheet (aka 18x13) = 234 square inches

8-inch round = 50.24 square inches

9-inch round = 63.59 square inches

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Scale-down

13x9 → 9x9: multiply by 0.69 (⅔)

13x9 → 8x8: multiply by 0.54 (½)

9x9 → 8x8: multiply by 0.79 (1, aka keep the recipe the same for a slightly taller bake, or use ⅔ for just scant)

9-inch round → 8-inch round: multiply by 0.79 (same as above)

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Scale-up

8x8 → 9x9: multiply by 1.26 (1 ¼)

8x8 → 13x9: multiply by 1.82 (2)

9x9 → 13x9: multiply by 1.44 (1 ½)

13x9 → half sheet pan: multiply by 2

8-inch round → 9-inch round: multiply by 1.26 (1 ¼)

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Change shape

9-inch round → 9x9 square: multiply by 1.27 (1 ¼)

8-inch round → 8x8 square: multiply by 1.27 (1 ¼)

8x8 square → 8-inch round: multiply by 0.79 (1, aka keep the recipe the same for a slightly taller bake, or use ⅔ for just scant)

9x9 square → 9-inch round: multiply by 0.79 (same as above)