A pan diameter is one length; a recipe fills an area. The useful first estimate therefore divides the total base area of the pans you will use by the total base area named by the recipe.
Use target area divided by source area
For one round pan, area is π × diameter² ÷ 4. For a square it is side², and for a rectangle it is length × width. Multiply each base area by the number of identical pans before dividing. This prevents a two-layer recipe from being compared with only one layer by accident.
Apply one factor only to scalable quantities
If the factor is 1.265625, multiply written ingredient quantities that scale linearly by 1.265625. Discrete eggs, leavening, spices, decoration, and package sizes may need recipe judgment rather than blind rounding. PanRatio can scale one sample number but does not rewrite or validate a recipe.
The area method assumes equal average batter depth
Equal base-area ratios preserve average batter depth only when the recipe behaves similarly and the usable pan geometry is comparable. Sloped walls, tubes, decorative moulds, different fill levels, and manufacturer capacity markings can break that assumption. When capacity is published, use it as an additional check.