Why pan area is what matters
A cake recipe is written for a certain amount of batter spread to a certain depth. When you switch pans, the goal is to keep that depth, so the batter has to grow or shrink with the area of the pan’s base:
- Round pan: area = π × diameter² ÷ 4
- Square pan: area = side²
- Rectangular pan: area = length × width
- Scaling factor = new pan area ÷ original pan area
Multiply every ingredient, including eggs, leavening and salt, by that factor.
Worked example
Your recipe is written for a 9-inch round pan and you want to bake it in a 9 × 13 inch sheet pan. The recipe uses 3 eggs.
- Original area: π × 9² ÷ 4 = 64 sq in.
- New area: 13 × 9 = 117 sq in.
- Factor: 117 ÷ 64 = 1.84.
- Eggs: 3 × 1.84 = 5.5 eggs. Use 5 eggs plus about half a beaten egg, or round to 6 if the recipe is forgiving.
- A round pan with the same area as the 9 × 13 would measure about 12.2 inches across.
Common pan sizes
| Pan | Area | Compared with a 9-inch round |
|---|---|---|
| 8-inch round | 50 sq in | 0.79 |
| 9-inch round | 64 sq in | 1.00 |
| 8-inch square | 64 sq in | 1.00 |
| 9-inch square | 81 sq in | 1.27 |
| 11 × 7 inch | 77 sq in | 1.21 |
| 9 × 13 inch | 117 sq in | 1.84 |
| Half sheet (18 × 13) | 234 sq in | 3.68 |
Tips for a good result
- Fill pans about two-thirds full for butter cakes, and no more than half for cakes that rise a lot.
- Watch the time, not the clock. A batter of the same depth bakes in roughly the same time, but large pans can brown at the edges before the center sets. Lower the oven by 25 °F for very large or deep pans.
- Measure inside the rim. Pan sizes are often stamped from the outside edge.
- Odd egg amounts: weigh beaten eggs; a large egg is about 50 g out of the shell.
- Bundt and tube pans are harder to compare by area; measure their volume with water instead and scale by volume.