When scaling up a recipe, multiply the ingredients by the pan’s base area and keep the batter at the same depth as before. If you do this, the baking time changes very little because the distance heat must travel to the center stays the same. If you increase the depth too, the time grows disproportionately, and the edges dry out while the center remains raw.
What determines how long batter takes to bake is not its weight, but the distance heat must travel from the surface to the center. Heat moves layer by layer; as each layer transfers heat to the next, the amount transferred decreases, so the progress is not at a constant rate and the same distance takes longer as it moves toward the center. The moment a bake is considered done is when the center, not the outside, reaches the desired temperature. The scaling rule follows directly from this. That is why pan selection is a decision about timing, not just appearance.
The rule is this: when scaling up a recipe, increase the ingredients in proportion to the pan’s base area while keeping the batter’s height the same. When moving from a 20 cm pan to a 24 cm pan, the area increases by 1.44 times, and the ingredients are multiplied by that factor. The batter spreads over a wider surface but stays the same thickness; the center is just as far from the surface as before. As a result, the baking time changes very little, and the minutes given in the recipe remain valid. The only thing that changes on the oven rack is the area the pan occupies.
If you increase the depth too, the equation breaks down. When the batter’s thickness doubles, the distance to the center also doubles, but the time grows by noticeably more than double because the progress slows as it goes. Meanwhile, the outside surface quickly approaches the oven’s temperature and keeps baking. The result is familiar: the edges are dark and dry while the center is still wet. That is why the first thing to consider when choosing a pan is not its diameter, but how high the batter will rise. The same volume bakes safely in a shallow, wide pan but causes problems in a narrow, deep one.
In practice, this means scaling up by choosing a wider pan. If you only have a deeper pan, there are two ways to compensate. The first is to lower the temperature somewhat and extend the time; this lets the outside surface heat more slowly and gives the center time to catch up. The second is to divide the batter between two pans; this best preserves the texture because it does not change the depth assumed by the recipe. Whichever you choose, the recipe’s stated minutes are now an estimate, and checking the center takes their place. Dividing it between two pans also speeds cooling.
The same rule works when scaling down, and it is less widely known in that context. If a recipe is cut in half and poured into the same pan, the batter becomes thinner, the center moves closer to the surface, and baking takes less time; if the pan is also made smaller, the depth is preserved and the time stays nearly the same. In the 2 directions, remember this one sentence: thickness determines the time, not the amount. Always check the center, and read the toothpick test by inserting it into the middle of the pan. Whatever color the edges are, make the decision based on the center.