A 9 inch round pan is not just one eighth larger than an 8 inch pan in the quantity that matters for equal batter depth. Its base area grows with the square of its diameter.
The Whatson Cake Pan Converter compares round, square and rectangular pans. It can preserve batter depth through an area comparison or compare entered batter volumes instead.
Equal depth gives a 1.2656 multiplier
Choose the area option, enter one round 8 inch pan as the original and one round 9 inch pan as the replacement. The checked multiplier is 1.265625, displayed as 1.2656.
The original total area is approximately 324.29 cm²; the new total area is approximately 410.43 cm². Dividing the new area by the original area gives the quantity multiplier.
For a recipe containing 200 g of an ingredient, the corresponding amount would be 253.125 g before practical rounding. Use Recipe Scaler in factor mode to apply the multiplier to a complete ingredient list.
Measure the inside, then count the pans
For a round pan, enter its internal diameter. A square uses its side length; a rectangle uses length and width. Select inches or centimetres beside the dimensions rather than relying on the unit in a product name.
The number of pans belongs in both sides of the comparison. Two replacement pans are a different capacity request from one replacement pan, even when their individual dimensions match.
Use volume mode when the batter depth changes
Volume mode includes the entered batter depth for each pan. This describes how deep the batter is intended to be, not necessarily the height of the pan wall.
For a general discussion of changing pan dimensions, see King Arthur Baking’s pan size guide. The Whatson calculation uses straight sided geometric shapes; it does not identify the exact capacity of a tapered or shaped mould.
The quantity ratio cannot choose the bake time
A recipe multiplier preserves the selected area or volume relationship. Pan material, batter behaviour and the changed shape still belong to the recipe method. The tool does not promise an equivalent baking time.
If you are starting with an empty pan rather than an existing recipe, Batter Volume Calculator estimates a filled volume from internal dimensions and your selected fill percentage.
Export the original and replacement measurements with the ratio. That record makes the calculation reusable when the same recipe is moved to another pan later.