Compare the footprints first
Two round pans 8 and 9 inches across differ by more than an inch of batter. The larger one has about 1.27 times the base area, so it takes about 27% more batter to fill to the same depth.
The calculation divides the new base area by the original base area. It assumes straight sides and enough height for the mixture to rise. Tube pans, fluted molds and sloping pie dishes need a separate capacity check. Pan material and batter depth can change baking time.
