Area, not diameter
A 9-inch round pan and an 8-inch square pan hold almost exactly the same amount — 63.6 versus 64 square inches. Comparing diameters would tell you the round pan is bigger, which is wrong. Capacity scales with area, so that is what the converter uses.
Reading the multiplier
New area ÷ old area gives the factor to multiply your batter by. Going from a 9-inch round to a 9 × 13 rectangle (63.6 → 117 in²) needs about 1.84× the batter. Going the other way, you make 54% of the recipe — or make the full batch and accept a thicker cake.
Depth changes time
- Same area, deeper pan: add 10–20% time and lower the temperature by 10 °C
- Same area, shallower pan: check 20% earlier — it will set fast
- Bigger area at the same depth: same temperature, slightly less time
- Always check at 70% of the calculated time the first run
Common substitutions that work
Two 8-inch rounds ≈ one 9 × 13 sheet for a layer cake. One 9-inch round ≈ one 8-inch square for brownies. A 9-inch springform ≈ a 9-inch round cake pan, but not a tube pan — the central heat core matters.
Frequently Asked Questions
Is a 9-inch round the same as an 8-inch square?
Almost exactly: 63.6 in² versus 64 in². Batter amounts swap directly.
How do I convert a tube pan?
Use the outer diameter and accept that the central tube changes baking — tube-pan recipes rely on it.
Do I need to change the oven temperature?
Only for depth changes; for area changes keep the temperature and adjust time.
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