What this calculator does
A cake tin conversion calculator scales a recipe written for one tin size or shape so that it fills a different tin correctly. Recipes are written for a specific tin, and swapping to a bigger or smaller one without adjusting quantities leaves the cake either too thin and overbaked, or too thick and undercooked in the middle.
The reliable way to convert is by area, not by a single dimension like diameter or width. Cake batter fills a tin to roughly the same depth regardless of its shape, so a tin with twice the base area needs roughly twice the batter. Comparing diameters alone is misleading, because area grows with the square of that measurement, not in a straight line with it.
The formula
Work out the base area of both tins, round tins use π × radius², square or rectangular tins use width × length, then scale each ingredient quantity by the ratio of the target tin's area to the original tin's area.
| Term | Meaning |
|---|---|
| Area ratio | Target tin area divided by original tin area; the factor every ingredient quantity is multiplied by. |
| Round tin area | π × (diameter ÷ 2)², using the tin's internal diameter. |
| Square or rectangular tin area | Width × length, using the tin's internal dimensions. |
The inputs explained
| Field | What to enter |
|---|---|
| Original tin shape | The shape of the tin the original recipe was written for. |
| Original tin diameter (if round) (cm) | Internal diameter of the original tin, if it is round. |
| Original tin width (if square/rectangular) (cm) | Internal width of the original tin, if it is square or rectangular. |
| Original tin length (if square/rectangular) (cm) | Internal length of the original tin, if it is square or rectangular (equal to width for a square tin). |
| Target tin shape | The shape of the tin you actually want to bake in. |
| Target tin diameter (if round) (cm) | Internal diameter of the target tin, if it is round. |
| Target tin width (if square/rectangular) (cm) | Internal width of the target tin, if it is square or rectangular. |
| Target tin length (if square/rectangular) (cm) | Internal length of the target tin, if it is square or rectangular. |
| Ingredient quantity in the original recipe (g) | The quantity of one ingredient in the original recipe, in whatever unit the recipe uses. |
When to use it
Only owning a different tin size
A recipe calls for a 20cm round tin but the only one on hand is 23cm. Scaling every ingredient by the area ratio keeps the batter depth, and therefore the baking time, close to what the original recipe expects.
Converting between round and square tins
A round and a square tin of similar size do not hold the same amount of batter, since a square fills its corners while a circle does not. Comparing their areas directly, rather than assuming a same-width swap is equivalent, avoids a tin that overflows or one left half full.
Scaling a recipe up for a bigger gathering
Moving from a small tin to a noticeably larger one for a bigger crowd works the same way: the area ratio scales every ingredient consistently, so the cake's proportions stay the same, just a larger version of it.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Scaling a 300g flour quantity from a 20cm round tin to other round tin sizes
The same 300g of flour from a 20cm round tin, rescaled for a range of other round tin diameters.
| Target round tin diameter | Scaled ingredient quantity | Area ratio |
|---|---|---|
| 15cm | 168.8 (same unit as entered) | 0.56× |
| 18cm | 243.0 (same unit as entered) | 0.81× |
| 20cm | 300.0 (same unit as entered) | 1.00× |
| 23cm | 396.7 (same unit as entered) | 1.32× |
| 25cm | 468.8 (same unit as entered) | 1.56× |
| 28cm | 588.0 (same unit as entered) | 1.96× |
Converting a 20cm round tin recipe to square tins of various sizes
The same 300g of flour from a 20cm round tin, rescaled for a range of square tin sizes.
| Target square tin side | Scaled ingredient quantity | Area ratio |
|---|---|---|
| 15cm | 329.5 (same unit as entered) | 1.10× |
| 18cm | 395.3 (same unit as entered) | 1.32× |
| 20cm | 439.3 (same unit as entered) | 1.46× |
| 22cm | 483.2 (same unit as entered) | 1.61× |
| 25cm | 549.1 (same unit as entered) | 1.83× |
Questions
Why not just compare the tins by diameter or width?
Because the amount of batter a tin holds depends on its base area, and area grows with the square of a linear measurement. A 25cm tin is only 25% wider than a 20cm tin, but its area, and the batter it needs, is about 56% greater.
Does this also work for tin depth or height?
This calculator scales ingredient quantities by base area, assuming both tins are a similar depth so the batter fills to a similar height. If the target tin is notably shallower or deeper than the original, the baking time will need adjusting even after the quantities are scaled correctly.
What if my recipe uses a bundt or unusual-shaped tin?
Treat it as approximately round using its overall diameter, keeping in mind that the true batter-holding volume of a bundt or other moulded tin is somewhat less than a plain round tin of the same diameter, since part of the centre is taken up by the mould.
Should I adjust the baking time as well as the quantities?
Yes, but that adjustment is more of a judgement call than a fixed calculation. A larger, shallower tin bakes faster and a smaller, deeper tin bakes slower, so start checking the cake earlier than the original recipe's time and use a skewer test rather than the clock alone.
For working out the area of an unusual tin shape directly, see the rhombus area calculator.