Discussion
Scaling up cake recipe
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I am thinking of making Ottolenghi's Apple Olive Oil cake for Rosh Hashanah next week. The recipe makes one 8 inch (20cm) cake that is split in half and frosted between layers and on top. I want to make this recipe in a 9x13 inch pan, but I think that original recipe would make too thin a cake. Would I double the recipe or make 1.5 times?
Anyone have experience of adapting/scaling such quantities?

Baking gets weird when you start scaling recipes up and down, for such an occassion (vs experiment on weekend alone) i recommend you follow the recipe exactly using the pan size asked for and make the original recipe twice
Cakes that go from round to rectangle can end up raw in the middle overcooked on the edges, or its too heavy and sinks, etcetc. Aka holiday meal crisis.0 
I had to do exactly this last week in making a celebration cake. I found this website useful, although the quantities are for a fruit cake, it gives the basic formula too.
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re: jpc8015
I agree. I would double the recipe. I have doubled and 1.5'd many cake recipes for different sizes and shapes of pans and never had any trouble with rising, over/undercooking. If it looks like it will cook to fast on the edges and to slow in the middle, I double pan (set the pan with the cake in another empty pan...do this for cookies and scones too if they are getting too brown on the bottom).
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re: jpc8015
Agreed as well. There are many cake recipes that say they make enough for two 8" round pans, or one 9" by 13", or 24 cupcakes, etc. So don't think doubling this one for a 9" by 13" pan will cause any problems. It's not like trying to scale it 4 or 5 times. Baking time may be a little longer, and Sooeygun's doublepanning suggestion is a good one if the edges are browning too quickly.
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re: jpc8015
Yep  double it. I have a chocolate cake that I sometimes make in 8" (or 9") layers or else in a 9x13. The thickness is about the same so should work fine.
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re: jbsiegel
for items like brownies/blondies/crumbles, that do not require lots of rise, i double without fear. not all cakes are that forgiving. it really depends on the recipe.
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If you plan to still split in half and frost layers, consider making two 8 inch cakes for easier handling. If the rectangular shape is important make in 8x8 pans and place sidebyside for final frosting.
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I just did a little MATH!?! Assuming pans are the same depth (say 2"), the larger one is a little over 1.8 times volume of smaller one. NOT much into baking, but would probably just double recipe, but have a small (loaf) pan ready cuz I don't think all the batter will fit... without ricking a mess in oven.
OOPS! I did my calculations on an 8" square pan... not pir2!!?
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http://www.joyofbaking.com/PanSizes.html
an 8" round holds 6 cups of batter. a 9x13 holds 14 cups. even doubling the recipe, the cake will be too thin and you may still wind up with overcooked sides and undercooked center.
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Assuming the 8" cake is approximately 2" in height, then the volume is (pi) * r^2 * height = (pi) * (16) * 2 which is approximately 100.48 cubic inches
The 9x13" pan has an area of 117, so for a height of 2", then volume would be 234 cubic inches.Doubling the initial recipe would give you a height of slightly less than 2". I'm a little more anal, and personally would do slightly more than double, but I would TOTALLY recommend that someone else (including you) just double it. Also, lower the oven temp by 25 degrees if you want to be sure the outside doesn't cook too much more quickly than the inside.
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