Cake Pan Size Conversion: Scale a Recipe to Your Tin

Find your cake pan conversion factor

Measure the inside of the pans at the base. Use the same unit for both pans: centimetres with centimetres, or inches with inches. Choose round, square or rectangular dimensions and compare the base areas. This calculation is intended to keep the same batter depth in the new pan.

Conversion factor = new pan base area ÷ original pan base area. New ingredient amount = original ingredient amount × conversion factor. A factor above 1 makes a larger batch; a factor below 1 makes a smaller one.

The formulas for round, square and rectangular pans

For a round pan, use the diameter rather than measuring across the outside rim or using the radius by mistake. For a rectangular pan, measure both its inside length and width. Keep a note of which dimensions you measured so the next bake is repeatable.

• Round to round: new diameter² ÷ original diameter². The π ÷ 4 terms cancel.
• Square to square: new side² ÷ original side².
• Changing shape: calculate both areas separately, then divide the new area by the original area.

Example: change a 20 cm round cake to 24 cm

The factor is 24² ÷ 20² = 576 ÷ 400 = 1.44. The diameter has increased by 20%, but the base area has increased by 44%. Multiplying the ingredients by 1.2 would leave the layer shallower than intended.

If the original batter uses 200 g flour, use 288 g. If it uses 150 g sugar, use 216 g. Apply the same factor to the other batter ingredients, keeping the calculation unrounded until you weigh the final amounts.

Example: a round pan to a square pan

A 20 cm round pan has an area of π × 20² ÷ 4, or about 314.16 cm². A 20 cm square pan has an area of 20 × 20 = 400 cm². The factor is 400 ÷ 314.16, or about 1.273. The same number on the pan label does not mean the same capacity for a layer of equal depth.

For a 20 × 30 cm rectangle, the area is 600 cm². Moving that batter to a 24 cm round pan gives 452.39 ÷ 600 ≈ 0.754. You would use about 75.4% of each batter ingredient. Keep enough working precision in your calculation, then round according to what your scale can measure.

Quick table for round cake pans

These factors keep the same intended layer depth. The final column shows the new amount for an ingredient that weighed 100 g in the original recipe. Multiply your own ingredient weights by the factor rather than using that last column as a complete recipe.

How to scale eggs and small ingredients

If the recipe states whole eggs and the new amount is fractional, beat an appropriate number of eggs until uniform, weigh them without shells and take the calculated amount. For a recipe that separates eggs, scale the whites and yolks separately. Do not replace a calculated fraction with a whole extra egg without recognising that the formula has changed.

Small amounts of baking powder, bicarbonate of soda and salt still need attention. A kitchen scale that reads only whole grams may not resolve a very small scaled quantity well. If accuracy becomes impractical, choose a batch size that can be weighed reliably rather than guessing spoon fractions.

Understand why the leavening ingredients are not interchangeable.

What the conversion factor does not tell you

This is an ingredient quantity calculation, not a new baking schedule. Pan material, actual batter depth, oven behaviour and the distance from the edge to the centre all affect baking. Keep the recipe’s temperature as your starting point and check doneness for the particular cake; do not multiply the baking time by the ingredient factor.

Do not fill the new pan higher merely because it has taller sides. Leave the headroom required by the recipe. Tapered pans, Bundt tins, domed moulds and pans with a central tube need a different capacity assessment because a simple base-area comparison does not describe their full shape.

The factor is useful for batter and for a flat filling layer at unchanged thickness. A covering cream or icing also has to coat the cake’s sides, so its required amount does not always scale by base area alone. Decoration and the number of portions are separate decisions.

Scaling a batch across several pans

Compare the total base area of all pans in the original recipe with the total area of the pans you want to fill. For example, two 20 cm round pans have the same total area as two separate 20 cm calculations. Moving from two 20 cm pans to three 18 cm pans gives (3 × 18²) ÷ (2 × 20²) = 1.215, so the full batch needs 1.215 times each ingredient for the same intended depth.

Mix only as much batter as your bowl and oven can handle properly. A mathematically correct large batch can still lose quality if it waits too long before baking or exceeds the mixer’s useful capacity. Record the pan, batter weight and actual bake result for the next time.

Apply the calculation to a chocolate sponge recipe.

Plan the syrup separately once the layers are baked.