To convert a recipe from one pan to another, you scale the ingredients by the ratio of the pans' areas. That's the whole formula — it's exact, it's just geometry, and once you see it you'll never trust a vague “use a bigger pan” again. Here's how it works, with the one thing area math genuinely can't tell you.
The pan conversion formula
A recipe is sized for a certain amount of batter at a certain depth. To fill a different pan to that same depth, you scale every ingredient by how much bigger or smaller the new pan's surface area is:
So you need each pan's area. For a round pan, area is π × radius² (the radius is half the diameter). For a square or rectangular pan, it's simply length × width. Multiply by the number of pans if you're using more than one.
A worked example: 8-inch round to 9×13
Say a recipe is written for an 8-inch round pan and you want to bake it in a 9×13 instead. Work out both areas, then divide:
The 9×13 has about 2.3 times the area, so you multiply every ingredient by 2.33 to fill it to the same depth. Going the other way — from the 9×13 down to the 8-inch round — you'd divide instead, scaling by about 0.43.
Common pan areas (reference chart)
These are the areas of the most common baking pans, computed with the formulas above. To convert between any two, divide the area you want by the area you have.
| Pan | Area |
|---|---|
| 8-inch round | 50.3 in² |
| 9-inch round | 63.6 in² |
| 10-inch round | 78.5 in² |
| 8-inch square | 64.0 in² |
| 9-inch square | 81.0 in² |
| 9×13 rectangle | 117 in² |
| 9×5 loaf | 45.0 in² |
What the area formula can't tell you
Here's the honest limit. The area math is exact for the ingredients, but it doesn't fix bake time and temperature — and those follow batter depth, not area. If you scale the recipe to keep the same depth, time and temperature barely change. But if you keep the same batter and just move it to a different pan, the depth changes, and that's where judgement comes in:
- Deeper batter (smaller pan, same batter): lower the oven about 15°C (25°F) and bake longer, so the centre cooks before the edges over-brown.
- Shallower batter (bigger pan, same batter): keep the temperature but start checking early — it bakes faster.
The pan size converter does the area math for any pair of pans and shows this depth-and-temperature reasoning step by step — including a “view formula & calibration” breakdown so you can check every number.
Sources and method: see our methodology and references.