The recipe wants a round springform of 26 cm, but all you have is a 20×20 cm square tin. Simply keeping the ingredients as they are risks a cake that is too tall or too flat. The Convert baking tin (round ↔ rectangular) does the maths for you and instantly gives the right factor for your ingredients.
Why the Base Area Matters
What counts when converting is not the diameter or the edge length, but the base area of the tin. If the batter height stays the same, the ingredient amount has to grow or shrink in the same ratio as the area. The formula is refreshingly simple:
Conversion factor = target area / source area
Calculate the areas like this: a round tin via π·(d/2)², a rectangular tin via length × width. Then multiply every ingredient by the factor.
Example: From Round 26 cm to Square 20×20 cm
A round tin of Ø 26 cm has an area of π·(13)² ≈ 531 cm². A square tin of 20×20 cm comes to 400 cm². So the factor is 400 / 531 ≈ 0.75. Every ingredient is multiplied by 0.75 – 200 g of flour become 150 g, 4 eggs become roughly 3. Because the square tin is a little smaller, the batter height stays similar; the smaller amount tends to bake through a touch faster.
Table of Common Baking Tins
For quick reference, the base areas of popular tins:
| Shape | Size | Area |
|---|---|---|
| round | Ø 18 cm | ≈ 254 cm² |
| round | Ø 20 cm | ≈ 314 cm² |
| round | Ø 24 cm | ≈ 452 cm² |
| round | Ø 26 cm | ≈ 531 cm² |
| round | Ø 28 cm | ≈ 616 cm² |
| rectangular | 20×20 cm | 400 cm² |
| rectangular | 20×30 cm | 600 cm² |
| loaf tin | 25×11 cm | ≈ 275 cm² |
Keep an Eye on the Baking Time
The conversion only affects the amount. If a flatter tin reduces the batter height, the cake bakes faster – check earlier with a skewer. If the height stays the same, you can stick to the original time. For any shape and size, the Convert baking tin (round ↔ rectangular) works out the factor in seconds.
