The rule: compare areas
To keep the cake the same height, the amount of batter has to grow with the pan’s area, not its diameter. That’s why a 10" pan doesn’t need 25% more batter than an 8" pan, but about 56% more.
- Round pan: area = 3.14 × radius × radius (the radius is half the diameter).
- Square pan: area = side × side.
- Rectangular pan: area = length × width.
Then divide the area of your pan by the area of the recipe’s pan. The result is the number you multiply every ingredient by.
Step-by-step example
The recipe is for an 8" round pan and you have a 9" round:
- Recipe pan: 3.14 × 4 × 4 = 50 in².
- Your pan: 3.14 × 4.5 × 4.5 = 64 in².
- 64 ÷ 50 = 1.27. If the recipe calls for 2 cups of flour, you’ll use about 2½ cups.
Common pan conversions
| Recipe pan | Your pan | Multiply by |
|---|---|---|
| 8" round | 9" round | ×1.27 |
| 8" round | 10" round | ×1.56 |
| 9" round | 10" round | ×1.23 |
| 9" round | 8" × 8" square | ×1.01 |
| 8" × 8" square | 9" × 9" square | ×1.27 |
| 9" × 13" pan | 8" × 8" square | ×0.55 |
| Two 9" rounds | 9" × 13" pan | ×0.92 |
| 9" × 13" pan | 10" × 15" jelly roll | ×1.28 |
A 9" round and an 8" square hold almost exactly the same amount of batter, so you can swap them without changes.
Baking time and temperature
- Same batter depth: the time barely changes; start checking a few minutes early.
- Deeper batter (smaller pan or extra batter): lower the oven about 25 °F and bake longer so the center cooks before the edges burn.
- Thinner batter: it bakes faster. Start checking halfway through.
- Always test with a toothpick: if it comes out clean, it’s done.
What about the eggs?
If you end up with a fraction, round to the nearest egg. For more precision, beat one egg and use only the part you need: a large egg is about 50 g (3½ tablespoons) without the shell.