How to Substitute Any Baking Pan for Another
Almost every baker has stood in the kitchen holding a recipe that calls for a 23cm round pan while owning only a 20cm square one, and then made the same reasonable-sounding mistake: comparing the numbers 23 and 20, deciding the difference is small, and baking anyway. The result is a cake that overflows, or one that bakes flat and dry, and the reason is that pans are two-dimensional surfaces while their stated sizes are one-dimensional lines. Twenty-three centimetres across is not 15% more pan than twenty centimetres — the honest comparison is between areas, and there it turns out the round pan holds slightly more than the square, not less.
The arithmetic is short. A round pan's area is π times the square of half its diameter, so a 23cm round gives π × 11.5² ≈ 415 cm². A square pan's area is simply side times side, so a 20cm square gives 400 cm². Those two are within 4% of each other, which is why the swap works with essentially no adjustment — a fact you would never guess by comparing 23 to 20. Meanwhile a 20cm round is only 314 cm², a full 21% smaller than the 20cm square that shares its stated size. The calculator above runs this comparison for you across round, square, rectangular and loaf shapes, and returns the number that actually matters: the multiplier to apply to your ingredients.
Common Pan Areas at a Glance
The table below lists the pans most home kitchens own, with the surface area of each and the batter capacity assuming a typical 5cm fill. Read down the area column rather than the size column and the real relationships between these pans become obvious immediately.
| Pan | Inches | Area | Batter Capacity |
|---|---|---|---|
| 15cm round | 6″ | 177 cm² | ~0.9 L |
| 20cm round | 8″ | 314 cm² | ~1.6 L |
| 23cm round | 9″ | 415 cm² | ~2.1 L |
| 25cm round | 10″ | 491 cm² | ~2.5 L |
| 20cm square | 8″ | 400 cm² | ~2.0 L |
| 23cm square | 9″ | 529 cm² | ~2.6 L |
| 33 × 23cm sheet | 13 × 9″ | 759 cm² | ~3.8 L |
| 21 × 11cm loaf (900g) | 8.5 × 4.5″ | 231 cm² | ~1.6 L (deeper) |
| 12-cup muffin tin | standard | ≈ 400 cm² | ~1.1 L |
Depth Is What Really Controls the Bake Time
Once you have matched the volume, the second half of the problem begins. Heat enters a cake from the outside and has to travel inward, and the distance it must travel is the depth of the batter, not the width of the pan. Pour the batter for a 20cm round into a 33 × 23cm sheet pan and the same volume spreads into a layer less than half as deep. That thin layer sets in a fraction of the original time, and if you leave it in for the stated forty minutes you get something closer to a biscuit than a cake.
The relationship is inverse and exact: depth changes by the inverse of the area ratio. Double the area and the depth halves. The calculator above computes the resulting batter depth for you and translates it into a practical instruction, because the two failure modes need opposite corrections. When batter sits deeper than the original, the outside sets long before the centre does, so you drop the oven temperature by around 10°C to slow the crust while extending the total time. When batter sits shallower, the temperature stays where it was and you simply start checking much earlier — a thin layer wants the same heat for less time, not gentler heat.
The Swaps Worth Memorising
A handful of substitutions come up so often that they are worth knowing by heart. The pattern behind all of them is that a square pan holds roughly 25% more than a round pan of the same stated size, because you are trading the corners back in.
| Recipe Calls For | Use Instead | Multiplier | Time Adjustment |
|---|---|---|---|
| 23cm round | 20cm square | 0.96× | None — near-perfect swap |
| 20cm round | 20cm square | 1.27× | Same, or scale batter up 25% |
| Two 20cm rounds | 33 × 23cm sheet | 1.21× | Cut 5–10 min; layer is thinner |
| 23cm square | Two 900g loaf tins | 0.87× | Add 10–15 min; much deeper |
| 23 × 33cm sheet | 12-cup muffin tin | 0.53× | 18–22 min instead of 35–40 |
| 25cm round | 23cm square | 1.08× | None worth making |
Where Pan Material and Colour Enter the Picture
Geometry gets you most of the way, but the pan itself is not thermally neutral. A dark, non-stick pan absorbs radiant heat far more readily than a shiny aluminium one and can brown the base of a cake ten minutes ahead of schedule; dropping the oven 10–15°C compensates. Glass and ceramic are poor conductors that heat slowly and then hold that heat stubbornly, so they typically want an extra five to ten minutes and benefit from being pulled slightly early to allow for carryover. Thin, warped aluminium sheets conduct quickly but distribute unevenly, which is why heavy-gauge pans are worth their price for anything that bakes longer than half an hour.
Silicone is the outlier. It insulates rather than conducts, so batter in a silicone mould relies almost entirely on hot air rather than contact heat. Expect noticeably less browning on the sides and base, longer total times, and a real benefit from setting the mould on a preheated metal baking sheet to give it a conductive floor to work against.
How to Use the Converter Above
Set the left panel to the pan the recipe specifies, including how deep the batter sits in it — 5cm is a safe assumption for most cake recipes if the author does not say. Set the right panel to the pan you actually own, and enter the height of its walls rather than the batter depth, since that is what determines whether the batter will fit. The three summary figures then tell you everything: the ingredient multiplier to apply to every quantity in the recipe, the percentage difference in surface area, and the depth your batter will reach in the new pan.
Multiply every ingredient by the multiplier, including eggs — for a 1.27× conversion, three eggs becomes 3.8, which in practice means three whole eggs plus one lightly beaten and measured out to roughly 80% of its volume, or simply four eggs with a tablespoon or two of extra flour to compensate. The overflow warning appears automatically whenever the resulting batter depth exceeds three-quarters of the pan's wall height, which is the point at which rise becomes a genuine risk rather than a theoretical one.
Frequently Asked Questions
Our Recipe Portion Scaler applies it across every ingredient at once and keeps leavening ratios intact.