An 8-inch square pan has 8 × 8 = 64 square inches. A 9 by 13 inch pan has 117 square inches. Divide 117 by 64 to get 1.828, so 200 g flour becomes 366 g and 150 g sugar becomes 274 g if the aim is similar batter depth. A recipe that already makes two 8-inch layers needs a different comparison: count the total area of both original pans before scaling.
Worked example with numbers
Start by confirming that pan type and depth suit the recipe. Calculate new area divided by original total area, multiply weight ingredients by that factor, and round only to a scale resolution that the recipe tolerates. The recipe scaler keeps a written ingredient list proportional. Beat eggs and weigh the liquid when a factor produces 2.7 eggs rather than silently using two or three.
How to make the calculation
Fill a cake pan roughly half to two-thirds full unless the original recipe specifies another level. A wider replacement makes a shallower layer, so begin checking before the stated finish time; a smaller or deeper pan makes a thicker layer and can require longer. Use the recipe’s centre test, not the clock alone. Record pan dimensions, batter mass, oven temperature, and actual bake time for the next use.
Check the units and assumptions
Do not scale flour and leave leavening, salt, fat, or liquid unchanged. Do not divide a 9 by 13 recipe between two small rounds without checking their combined area and depth. Do not assume that a pan with the same volume has the same exposed surface, or that glass, ceramic, dark metal, and light metal brown identically. Do not turn a baked-goods calculation into a food-safety test; follow the recipe’s handling instructions.
Common mistakes
This method is for recipes whose ingredient ratios can be scaled and whose intended thickness is not highly specialised. It is not a reliable substitute for a jelly roll pan, a very tall springform, a tube cake, a baked custard, or a recipe with a fixed package component. If the new pan is visibly too full, remove some batter rather than relying on an estimate to prevent overflow.
When this result does not apply
King Arthur Baking notes that a smaller pan can make a cake take longer and risk overflow, while a larger pan makes a shallower cake that should be checked earlier. Its pan-size guidance specifically distinguishes diameter from volume and recommends allowing headroom.
Source for the external fact
Compare round pans with the cake pan conversion calculator, scale a complete formula in the recipe scaling tool, and calculate flour-water balance with the dough hydration calculator when the recipe is bread-like.
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