The one swap worth memorising
An 8-inch square pan and a 9-inch round pan have almost exactly the same area — 64 square inches against 63.6. That is a difference under 1%, so you can swap them with no change to the recipe at all. It is the single most useful pan substitution there is, and most recipes never mention it.
Area or volume — the question this tool asks
Every other pan converter gives you one number. That number is wrong half the time, because scaling a recipe has two different goals and they need different maths.
Same batter depth (area ratio). The batter sits at the same thickness as the original, so it bakes in roughly the same time and browns the same way. The cake ends up the same height but there is more or less of it. This is the right default.
Same amount of cake (volume ratio). You get the same number of servings, but the batter sits deeper or shallower. A deeper layer needs longer in a cooler oven; a shallower one can dry out.
If both pans are the same depth these two are identical and the tool doesn’t ask. It only asks when the answer actually changes.
Why a 12-cup Bundt pan doesn’t hold 12 cups of batter
The number stamped on a Bundt pan is the volume filled to the brim with water. Batter cannot go to the brim — it rises. Between the fluted sides, the central tube and the need to stop at two-thirds, a “12-cup” pan realistically takes about 6 cups of batter.
That is why this converter lists both numbers, and why it will not scale a Bundt by area. The area formula treats a fluted ring as a flat disc and overestimates badly. For Bundt pans the tool always uses usable volume.
Fill halfway to two-thirds — never more
This is the rule that saves ovens. Batter with any chemical leavening will climb, and a pan filled to three-quarters will go over the side. Two-thirds is the hard limit; half is more comfortable for a rich batter.
If the tool warns you about overflow, don’t reach for a bigger multiplier — bake the surplus as cupcakes for about 18 to 22 minutes. They will be done long before the cake is.
About “lower the oven by 25°F”
You will read this advice everywhere for deeper pans. It is not settled. King Arthur Baking does not recommend it as a general rule, and America’s Test Kitchen argues against it explicitly, preferring to judge by visual doneness rather than change the temperature.
This tool therefore never suggests a temperature change by default. It only mentions the option when the new pan is more than 1.5 times deeper, and it labels it as optional. What it always gives you instead is a time adjustment, which is the part everyone agrees on: deeper means longer, shallower means check early.
One detail worth knowing if you do lower the oven: 25°F is a difference, not a temperature. Converted properly it is about 14°C, not −4°C. The unit converter has a checkbox for exactly this.
Working from a recipe in cups? Cups to grams by ingredient is the companion tool — a cup of flour and a cup of sugar are nowhere near the same weight.