Cake Pan Size Conversion: How Much More for 15 cm to 18 cm? A Chart for Round Pans

Round pans scale by the square of the diameter ratio

When you change pans, what you want to keep is the baked height (the thickness of the batter). For the same thickness, the batter you need is proportional to the pan's base area. A round pan's base area is radius × radius × 3.14, so when the diameter grows 1.2 times, the area grows 1.2 × 1.2 = 1.44 times.

So 15 cm to 18 cm is ×1.44, not "3 cm more, so ×1.2," and 15 cm to 21 cm is ×1.96, almost double. If you scale by feel from the diameter and bake 20% more batter in an 18 cm pan, the thickness only reaches about 83% of the original. In a recipe that slices a sponge into two layers, that difference directly turns into a cake that's too thin to cut.

Scaling chart for round pans (12–24 cm)

Rows are the recipe's pan and columns are the pan you want to use. For round pans of the same height, multiplying every ingredient by this factor gives the same thickness.

• The three most common combinations are 15→18 cm at ×1.44, 18→15 cm at ×0.69 and 18→21 cm at ×1.36.
• 18 cm to 12 cm is ×0.44, a little under half. 15 cm to 12 cm is ×0.64, almost two-thirds.
• Above ×2.5, the batter often won't fit your hand mixer's bowl, so making it in two batches is safer.

The relationship between cake size numbers and servings is covered in Cake Size Numbers and Servings, along with conversions to square pans.

Converting a sponge recipe

Here, a 15 cm whole-egg sponge (2 eggs, 60 g granulated sugar, 60 g cake flour, 20 g butter, 1 tbsp milk) is converted to 12 cm, 18 cm and 21 cm. Paste the ingredients into the Cake Pan Size Converter to rewrite them all at once, in the same form as this table.

Taking 2 eggs at 15 cm as the base, a sponge needs about 2.9 eggs at 18 cm, about 3.9 at 21 cm and about 5.1 at 24 cm. The figures often seen in recipes, 3 eggs for 18 cm and 4 for 21 cm, closely match this calculation.

When you end up with something like 2.9 eggs

Applying a factor usually leaves a fraction of an egg. Since eggs vary in size, weighing beaten egg is more reliable than rounding the count. A medium egg is about 50 g without its shell (about 20 g yolk and 30 g white), so for 2.9 eggs, use about 145 g of beaten egg.

1. 1

   Crack extra eggs and beat them

   Crack one more egg than you need, beat well, then weigh into a bowl.

2. 2

   Match the grams

   Use about 145 g for 2.9 eggs and about 65 g for 1.3 eggs. Leftover beaten egg can go into an omelet or glaze.

3. 3

   If you round up, adjust the rest too

   For a sponge, where whipping matters, you can round up to 3 eggs and bring sugar and flour to 3 ÷ 2 = ×1.5 (90 g). You get a little more batter, but the balance of the recipe holds.

Baking time and temperature when you change pans

When you convert between round pans at the same thickness, the batter's depth doesn't change, but the bigger the pan, the longer heat takes to reach the center.

• To test for doneness, insert a bamboo skewer in the center and see whether it comes out clean.
• If the top browns too soon, cover with aluminum foil and keep baking.
• Ovens vary a lot by model, so the first time, take a look 5 minutes before the recipe's time.

Pans of different heights, and moving to other shapes

The chart gives factors for round pans of the same height. If you bake a recipe for a 6 cm-high pan in an 8 cm-deep pan as it is, the same factor works; you just get extra room at the top. If you want a taller cake to suit the deeper pan, convert by the ratio of volume rather than base area.

• Square pans: you can convert by base area, as with round pans. Details are in Cake Size Numbers and Servings.
• Loaf pans and muffin pans: thickness doesn't line up, so convert by the amount of batter. See Loaf Pan Sizes and Conversions.
• Chiffon pans: height changes with diameter, so the factor is roughly the cube of the diameter ratio. See Chiffon Pan Sizes and Amounts.
• Baking trays (roll cake): between trays, convert by area. See Roll Cake Tray Size Conversion.

FAQ

How do I scale a 15 cm recipe for an 18 cm pan?

Multiply by 1.44. A round pan's base area changes with the square of the diameter ratio, so (18 ÷ 15)² = 1.44. A 2-egg recipe needs about 2.9 eggs (about 144 g of beaten egg).

What are the amounts when making an 18 cm recipe in a 15 cm pan?

About ×0.69. A sponge with 3 eggs, 90 g sugar and 90 g cake flour becomes 2.1 eggs (about 104 g), 63 g sugar and 63 g cake flour.

How many eggs does an 18 cm sponge take?

If the 15 cm version uses 2 eggs, 18 cm takes about 2.9, so about 3 eggs. 21 cm takes about 4, and 24 cm about 5.

If the pan's diameter doubles, do the ingredients double too?

No, you need 4 times as much. Area is proportional to the square of the diameter, so 12 cm to 24 cm is (24 ÷ 12)² = ×4. Doubling gives a cake half as thick.

How much longer should I bake in a bigger pan?

Keep the temperature the same or 10 °C lower, and bake 5–15 minutes longer. To finish, insert a bamboo skewer in the center; it's done when no batter sticks to it.