A recipe for a 24 centimetre cake yields thinner batter in a 28 centimetre tin if you pour in the unchanged quantity. To resize, you must compare the internal base area of the tin. If you also change the batter height, the volume ratio comes to the fore. Scaling ingredients is easily calculated, but baking time requires separate checking. Handling these two decisions separately makes the calculation usable in practice.
For round tins, the square of the diameter matters
The area of a circle is π × diameter² / 4. When comparing two round tins, π and the division by four drop out of the ratio. The new recipe multiplier is therefore new diameter² / old diameter². Resizing a 24 centimetre recipe to 28 centimetres gives a multiplier of 28² / 24² = 784 / 576 = approximately 1.3611.
This means approximately 36.11 per cent more batter at the same planned batter height. The simple ratio of diameters would only be 28 / 24 = 1.1667, which would give too few ingredients. The reason for the two different results is that the tin becomes larger in two horizontal directions.
An illustrative base recipe contains 300 grams of flour, 180 grams of sugar, 120 grams of butter and 200 grams of shelled eggs. The area-proportional version contains approximately 408.3 grams of flour, 245 grams of sugar, 163.3 grams of butter and 272.2 grams of eggs, respectively. This is an example of scaling four ingredients and is not a complete baking recipe on its own.
Internal size and batter height are important together
The diameter measured on the outer rim of the tin may differ from the useful internal diameter. For the calculation, measure the part into which the batter goes. For tins with strongly sloping sides or complex shapes, the base area approximation may be less accurate. In such cases, the actual filling volume belonging to the recipe or the manufacturer's capacity of the tin may provide a better starting point.
The total height of the tin is not the same as the height of the batter to be filled. You must leave free space appropriate to the recipe and the volume change of the batter. The King Arthur Baking guide on tin sizes shows with examples how baking changes with tin size. Therefore, keep the filling instructions of the specific recipe in mind even after resizing.
If you plan a starting batter layer of 4 centimetres instead of 3 centimetres at the same tin diameter, the volume multiplier is 4 / 3 = 1.3333. If the size changes simultaneously from 24 to 28 centimetres and from 3 to 4 centimetres, the product of the two ratios is 1.3611 × 1.3333 = approximately 1.8148. The higher batter layer can significantly alter the baking process, so a large deviation from the base recipe must be checked by trial.
The height in this multiplier is always the raw layer filled into the tin. If your wish concerns the height of the baked cake layer or the cake assembled with cream, this must first be linked to the actual filling and cream size data of the proven recipe. Do not write the centimetres of the finished cake directly in place of the raw layer. The notes should list the filled layer, the baked layer and the height of the cake assembled with fillings on separate lines; you plan the external covering from the latter.
From round tin to rectangular baking tray
The base area of a 24 centimetre circle is approximately 452.39 square centimetres. The area of a baking tray with internal dimensions of 30 × 20 centimetres is 600 square centimetres. For the same batter height, the resizing multiplier is 600 / 452.39 = approximately 1.3263. The base version containing 300 grams of flour would thus require approximately 397.9 grams of flour.
In the reverse direction, the old and new areas swap places in the division. If you switch from a 600 square centimetre tray to a 24 centimetre circle, the multiplier is approximately 0.7540. Less batter is needed for the smaller tin at the same height. Perform the back-and-forth conversion using the same pre-rounding ratio so that details do not drift.
For two smaller tins, add the areas together first. The total area of two 18 centimetre circles is approximately 508.94 square centimetres. This is exactly 1.125 times the area of the 24 centimetre tin. Thus, the two smaller tins require 12.5 per cent more batter at the same filling height. The King Arthur Baking article on alternative tin sizes also starts from the comparable surface for appropriate recipes.
Three different tins: the batter weighing list
Let's look at a separate, specific quantity example: a full, proven recipe's 1 200 g of mixed batter yields a 3 cm starting layer in a 24 cm tin. Now you want to fill two 18 cm and one 12 cm round tins with the same 3 cm layer. The internal sides of the tins are nearly vertical, and we assume the batter has the same composition and density in all three.
The ratio of the total new base area is (18² + 18² + 12²) / 24² = 1.375. The new recipe quantity is therefore 1 200 × 1.375 = 1 650 g. This must be distributed according to the areas of the three tins:
| Tin | Share of the total area of the three tins | Raw batter to weigh |
|---|---|---|
| First 18 cm | 324/792 | 675 g |
| Second 18 cm | 324/792 | 675 g |
| 12 cm | 144/792 | 300 g |
Dividing equally into three would give 550 g in each. Thus, a much thicker layer would form in the smaller tin, and a thinner one in the two larger ones than planned. The 1 650 g here is the mass going into the tins: allow separately for any extra batter needed to cover what remains in the mixing bowl.
If only the original 1 200 g is available, an area-proportional distribution list can also be made from it: 490.91 g, 490.91 g and 218.18 g. The common starting height will then be approximately 2.18 cm (3 / 1.375). This is already a different baking task; checking the recipe's doneness and the placement of the tins in the oven still matter. The 1.375 ratio cannot be transferred to baking time.
Calculate the external covering of the finished cakes separately later with each final diameter and height. In the fondant quantity planned from your own swatch (in Hungarian), you can also track the difference between the sheet to be rolled out and the covering remaining on the cake; the combined multiplier used for the batter does not resolve this.
How do you handle fractional eggs and small quantities?
If the recipe gives eggs in pieces, the shelled mass can be useful for more precise scaling. In the example, 200 grams of homogeneously mixed eggs multiplied by 1.3611 is 272.2 grams. The ratio can thus be measured, and you do not have to guess what the fractional piece count means. If the recipe treats the yolk and white separately, these must be scaled separately.
For small quantities, the scale's resolution is limited. You cannot check a theoretical 2.7 gram batch to the tenth of a gram with a device displaying in grams. Adjust the ingredients and the precision of measurement to the recipe. Also record the rounding point so that a later repetition starts from the same result.
For baking powder, whipping, mixing technique and resting, the ingredient ratio is only one part of the task. A larger mixture may require a different vessel or more mixing cycles. If you exceed the machine's capacity, it may modify the preparation result even at the same total mass. Therefore, re-read the recipe's workflow during resizing.
Do not multiply baking time by the recipe multiplier
A 36.11 per cent larger batter quantity does not imply 36.11 per cent longer baking. Heat transfer depends on the depth of the batter, the shape and material of the tin, and the oven's operation. At the same height, a wider tin means a different situation than a much higher batter layer at the same diameter.
Treat the baking time in the original recipe as a starting point for checking the resized cake. The cake-tin converter does not calculate a baking-time range. Check doneness using the observations specified for the particular recipe. Sponge cake, brownies and baked cheesecake need different final textures, so a single general time or test cannot be applied to all of them.
If the recipe gives the oven temperature in Fahrenheit, the oven temperature converter (in Hungarian) helps with the unit. This numerical conversion alone does not resolve the technological difference between fan-assisted and top-bottom heating. Choose the baking mode according to the recipe and your own device's instructions.
How to work with related calculators
Use the cake-tin converter to compare two round tins with the same batter depth. For rectangular tins, different shapes or a changed batter depth, calculate the area or volume ratio separately using the examples above. Apply the resulting multiplier in the recipe scaler to each ingredient in turn: changing from 24 to 28 centimetres gives the exact multiplier 49 / 36, so you can enter 36 original and 49 new proportional units. These units express the ratio; decide the actual number of slices separately.
Different multipliers for filling and external cream
For a cake increased from 24 to 28 cm, the multiplier for the filling between the cake layers is also 49/36 if the number, thickness and composition of the filling layers remain unchanged. For the external cream, a different surface must be covered. Let's look at a specific model for this: both finished cakes are 10 cm high, and only the top and side receive an identical thickness, uniform, thin cream layer. We are not calculating with decorative rosettes and separate borders now.
The old cake's top is 144π and its side is 240π square centimetres, together 384π. The larger cake's are 196π and 280π respectively, together 476π. π drops out of the ratio, so the quantity multiplier for the external cream is 476/384 ≈ 1.2396. In the approximation of the thin layer, this means a 23.96 per cent increase.
If 600 g of cream remains on the outside of the proven 24 cm version, approximately 743.75 g belongs to the same layer on the new cake. The 49/36 multiplier used for the batter would instead give 816.67 g, about 72.92 g more. The difference between the two plans is caused by the surface geometry.
Add the quantity needed for the cream remaining in the bowl and on the spatula separately based on your own work; the mass above refers to the layer going onto the cake. Recalculate if the height, thickness or decoration differs. The fondant quantity calculator provides separate size-based help for the rolled covering; the rolling and cutting surplus needed there differs from the surface ratio of smoothed cream.
What is worth noting down after the first bake?
Record the internal tin size, the mass of the filled batter, the filling height, the baking mode and the actual doneness time. If you baked in two tins, their placement is also part of the experience. For later repetition, these own data say more than a general diameter list.
Thus, successful resizing becomes a verifiable recipe variant. If only the diameter changes next time, you can start from a known batter height and your own baking experience. The calculator organises the quantities, and the notes preserve the practical conditions under which the cake was made.
What fits from the existing ingredients?
From a recipe using 300 g of flour for a 24 cm tin, 400 g of flour allows you to prepare 400 / 300 = 1.3333 times the original recipe quantity if the other ingredients are also available. At the same raw layer thickness, the diameter of a matching round tin is 24 × √(400 / 300), approximately 27.71 cm. Thus, the layer will be slightly thinner if you stay in the same tin compared to the 408.33 g of flour calculated for the 28 cm tin. The raw height is 400 / 408.33, roughly 97.96 per cent of the originally planned value. Baking time still cannot be calculated in the same ratio from this.
Final knowledge test
Choose one answer for each question. Each correct answer earns 1 point; an incorrect answer, a skipped question or more than one selected answer earns 0 points. You can score 0–3 points in total. Read the solutions after answering.
1. question: The diameter of the round tin increases from 24 to 28 cm, the raw layer remains the same. What is the recipe multiplier?
- A) 28 / 24, approximately 1.1667.
- B) Exactly 2.
- C) 28² / 24², approximately 1.3611.
2. question: You divide 1650 g of batter into two 18 cm and one 12 cm round tins for the same layer thickness. How much goes into the 12 cm one?
- A) 550 g.
- B) 300 g.
- C) 675 g.
3. question: A 1.3611 times recipe quantity was made. What can you do with the baking time?
- A) You check it based on the recipe and the actual baking; the mass multiplier alone is insufficient.
- B) You always leave it unchanged, regardless of the tin.
- C) You automatically multiply it by 1.3611.
Solutions and explanations
- 1. C. At the same height, the base area of the circle is proportional to the square of the diameter. If you chose otherwise, review this: area ratio of round tins.
- 2. B. The ratio of the areas is 324 : 324 : 144, from which the small tin's share is 144 / 792. This is 300 g from 1650 g. If you chose otherwise, review this: area-proportional distribution for multiple tins.
- 3. A. The tin, layer thickness and oven heat transfer all matter; the geometric recipe multiplier is not a baking time rule. If you chose otherwise, review this: limits of geometric scaling and baking observation.
What does your result mean?
- 0 points: start with the basic relationship: area ratio of round tins. Work through the related example in the article.
- 1 point: return to the explanation of the two missed questions and write down the correct inputs.
- 2 points: check the reference basis and conditions on the topic of the skipped or incorrect question, then recalculate.
- 3 points: you correctly applied the three examined relationships. Try it with your own data, and also note the limits of the geometric scaling and baking observation section.
Translated and adapted from the Hungarian article: Hungarian original.