13x18x2 Sheet Cake - Need Help?

I have never used a boxed cake mix so I have no idea how much 4 of them will make. You could use some simple arithmetic and extrapolate from the information on the box relative to the size of your pan.

You should plan to use the the full amount of eggs, water and eggs for each box of mix.

Because it's a sheet cake you should keep the time and temperature the same but keep a close eye on it at the 25 minute mark. If it looks like it's getting too brown lower the tempeature to 325. Use a toothpick to test for doneness and judge the time remaining from there.

I would line the sheetpan with parchment paper to make it easier to get the cake out.

Don't think a half-sheetpan (13 x 18) can hold a dozen egs and a pound of butter, let alone the water and mix.
Try for two boxes. Bake again if you have to.

Well if the volume of the sheet pan is 468 in^3 (13 x 18 x 2), then what you need to do is look at the directions on the box. If the box tells you that it makes two 8" cake rounds, then calculate the volume of two 8" cake rounds and divide 468 by that combined volume. That number will be how many boxes of cake mix you need to fill the 13x18x2 sheet pan. Granted, this assumes that the 8" cake rounds (or whatever pan they tell you to use) would actually be completely filled during baking, but it should still at least give you a rough estimate.

EDIT: Oh, I just found a box of Betty Crocker cake mix that's been lying around for who knows how long. The pan sizes listed on the back are 13"x9", two 8" rounds, two 9" rounds, cupcakes, and a fluted bundt pan. Based on the pictures on the box, if that can be any indication, the recipe fills the 13x9 cake pan, meaning you'd simply need two boxes of mix and all the ingredients.

If I were you, I'd just mix up the four batches of cake mix, fill the pan halfway with batter, bake the cake at the box temperature, and keep an eye on it, particularly around 25-30 minutes. When that's done baking, pull the cake out, cool it, take it out of the pan, etc. Then you can just load up another batch of cake so you end up with at least a double layer cake and you can put icing between the layers.

Sorry if this made things even more confusing, I'm a bit tired and unfit to be typing. Good luck!

Your correct its to much volume for a 1/2 pan:chef: and I know nothing about mixes in boxes?

Done this before. I used three boxes and used half the mix for each layer. If you only have one pan, thats okay the mix will hold up while one bakes. Use parchment paper and spray or grease the pan and paper well. This will give you a 1 to 1.5 inch layer. Cool, wrap and freeze the layers and you can handle them easier with less breakage. Mix cakes are very crumbly, and are better handled almost frozen. It won't hurt them and actually makes the icing of them easier.

Good Luck

Assuming the oven is not hugely energy deficient, cook times for large pieces (or cakes), are not an exact multiple of the volume or weright (mass) as compared to a smaller piece. A lot depends on thickness. I.e., the distance the heat has to travel to get to the center -- everything else being equal.

You're probably already aware that several small pieces cook in almost the exact time a single small piece cooks. Same thing.

That said, it's a fairly safe bet your large sheet will take a bit longer than a small sheet -- but only by a bit. Start watching the cake at the same time you would a smaller one just to be safe.

Nearly all boxed mixes are pretty much standard sized. Undoubtedly yours is one. In addition, most of them include a chart for the appropriate pan sizes. If yours doesn't, do a little googling and you'll find several charts that have it all worked out. For instance: Party Cake Baking Time and Batter Amounts - 2 in. Deep Pans.

Like most charts, the one I linked is built around the volume of the mix in cups (not cubic inches or centimeters). For what it's worth, a typical mix will get you between 4-1/2 and 5-1/4 cups volume. Also like most charts, you can drive yourself crazy if you worry about an some number of mix-boxes not meshing with the "standard" cake height on the chart. For instance, if the chart tells you that 11 cups are needed to make a 2" cake -- just round it off, use two boxes of mix, and settle for 1-3/4" - 1-7/8". The earth will stay in orbit, I assure you.

However ... you do want to watch out for over-filling the pan. At least if you actually want to keep the pan from overflowing. Your pan will give you a 2" cake with with roughly 7 cups of batter. So you're either going to bake a thin cake with one box, or bake 1-1/2 boxes. Up to you.

Don't forget to use a greased sheet of parchment for the bottom of the pan; one which overlaps the ends. It makes handling the cake after baking sooooooooooooooo much easier.

Hope this helps,
BDL