I have a 12 x 18 pan which I consider to be half a sheet. So if I need a full sheet, I bake two and put them together. That gives you 108 2x2 servings.
I would say 3 boxes is a good estimate for the 12 x 18 pan. I don't like to torte so if I am going to fill it, I usually bake two separate layers. I've done 2 boxes for each layer and it really ends up to be too tall - doesn't fit in a box. So 1.5 boxes of mix per layer if you are going to do a filling would work.
There are different opinions on what makes a full sheet, but most people have to join a couple of cakees to make it because as noted, the ovens aren't big enough. I would join together 2 of my 11x15 (2 mixes each -- I doctor them, so it doesn't take all the batter -- about 1 cup left over) or join 2 of my 12x18 for more servings (about 72). That would take 3 mixes each.
HTH -- S.
I did a full sheet cake just recently. I did (4) 9X13's because I don't have the heating core or bake even strips which are needed for a 12x18 pan. Anyway, a full sheet cake is around 25" x 18". Doing the four worked out well. I had to make four batches of buttercream to cover and decorate the cake. With all that buttercream, it gets quite heavy so make sure you have a sturdy base. The lady I made it for cut big pieces and it fed about 75 people. My picture is in "My Photos." Good luck.
Some people bake two 1/2 sheets - you have two different flavors if you want. Also, make sure you double or triple your board you need to have support for the middle. Also mention to the customer to be careful picking up cake it can split down the middle unless you use a good support base then it will be fine.
size
18"x26" (Full) - servings 117, cute 2x2x2
http://www.allinonebakeshop.com/cakes.asp
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here is some information to help you:
1/2 sheet to be 12 x 18 and you can get 40 - 48 pieces.
Full sheet pan Standard 24x16x2
3/4 sheet pan- 18X12X2
Half sheet pan Standard 16x12x2
Quarter sheet pan Standard 13x9
Quarter Sheets, Half Sheets, and Full Sheets Explained
The retail baking industry developed standard sizes for rectangular sheet cakes, beginning with their standard sheet pan size of 16 by 24 and dividing it into halves (half-sheet) or quarters (quarter-sheet). The manufacturers who supply paper and plastic items to the baking industry use these standard sizes to create a variety of cake boards, boxes, and domes.
In an effort to develop a unique market share, some manufacturers began promoting pans of slightly different sizes, selling mainly to home bakers through hobby and specialty stores. Since home bakers do not have the volume purchasing power of retail bakers, most of the paper and plastics manufacturers have not created boards or boxes to accommodate these sizes.
The bottom line: Boards and containers for sheet cakes will always be easier to find if you use the retail baking industry's standard size pans. You will save yourself a lot of frustration by sticking with the standards.
Baking and Paper Industry Standards; The sizes below are recognized as industry standards.
Quarter Sheet Cakes: 8 x 12 pan, 10 x 14 cardboard
Half Sheet Cakes: 12 x 16 pan, 13.5 to 14 x 18.5 cardboard
Full Sheet Cakes: 16 x 24 pan, 19 x 27 cardboard
Non-standard Sizes. These sizes are not standard sizes. Suitable boards and containers can be very difficult to find.
9 x 13 pan: We call it the large quarter size. Very few of the paper suppliers carry the boards (11 x 15) and boxes (12 x 16) for cakes made in 9 x 13 pans.
10 x 15 pan: Use the standard half sheet boards and boxes
11 x 15 pan: Use the standard half sheet boards and boxes
12 x 18 pan: We call it the large half size. This size pan will not fit in most home ovens. Wilton sells a set of 14 x 20 boards that can be used with this pan, but they sell no boxes. If you cannot locate a large half board (14 x 20) and box (14.5 x 20.5), you will have to use full sheet boards and boxes.
Same as most. I usually do two 12x18's and depending on the serving size, you'd get somewhere around 90 and 110 servings. I just let my clients know ahead of time that the cake is meant to be served as 1.5x2in slices. From there they can guage whether theylll need more cake or less cake based on the size they want to serve.
I refuse to use the terms "Half" and "full" and here's why:
http://forum.cakecentral.com/modules.php?name=Forums&file=viewtopic&p=3245473#3245473
Here's where I've done the math on servings per size:
http://forum.cakecentral.com/cake-decorating-ftopict-302303-feeding.html+80
I did a full sheet cake just recently. I did (4) 9X13's because I don't have the heating core or bake even strips which are needed for a 12x18 pan. Anyway, a full sheet cake is around 25" x 18". Doing the four worked out well. I had to make four batches of buttercream to cover and decorate the cake. With all that buttercream, it gets quite heavy so make sure you have a sturdy base. The lady I made it for cut big pieces and it fed about 75 people. My picture is in "My Photos." Good luck.
Not sure if you heard this before or not (i just learned it myself!) you can use a flower nail to get it cooked good in the middle. I usually spray mine really well with Pam and I haven't had any problems with the middle of the cake being cooked (and it's pretty level too) or the nail sticking to the pan. HTH. 
As for the sheet cake...haven't had to do a full sheet yet, so I'm not much help there...but would love to see the answer! Good luck!
I did a full sheet cake just recently. I did (4) 9X13's because I don't have the heating core or bake even strips which are needed for a 12x18 pan. Anyway, a full sheet cake is around 25" x 18". Doing the four worked out well. I had to make four batches of buttercream to cover and decorate the cake. With all that buttercream, it gets quite heavy so make sure you have a sturdy base. The lady I made it for cut big pieces and it fed about 75 people. My picture is in "My Photos." Good luck.
This is the first time i am seeing full sheet cake...wow...your cake is huge. Thanks for sharing the info with me. Hope everything goes good with my cake.
Well I use 1/2 sheet cake pan times 2 and I use 2 cake mixes for each cake....Of course I love to make one cake different from the other so that way the people get to choice So I normally make one side Choc and one side Van or Strawberry, etc. Like someone said before if you are going to torte...you should cook one cake mix at a time and then put the filling in the middle instead of trying cut the 1/2 sheet cake in half because it is so hard to get it off the without it breaking in half.
HTH and good luck.
Rajinaren, I just noticed you're from Austin. I'm in Round Rock. Anyway, I wanted to let you know that I got my board for that cake from All in One Bakeshop on 183. They said I could stand on it and it would hold. Sure enough, I had no problems. Plus, they have great boxes there for that size too. Best of Luck to you.
Thanks gdixoncakes! I live in austin and will be moving to Round rock this year end...since we are building a new house there. Thanks for the shop. I bought my cardboad and box there. I took Gumpaste and candy class there. They are good. Nice to hear from somebody in austin.
Keyshia, thank you too for the flower nail info. I had forgotten about that, but will try it. Thanks.
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