a 12 and a 9 would give you 87 servings according to earlenes chart
http://www.earlenescakes.com/ckserchart.htm
Here are all four Wilton cake preparation and serving charts:
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
These charts give batter requirements by pan size (for 2 & 3" deep pans) give baking times and temp.
Also gives servings per pan by size and by either party or wedding serving size.
This way you can determine how many servings are in each layered cake whether party or wedding size servings.
Please, no "save" posts per Jackie and Heath:
http://forum.cakecentral.com/cake-decorating-ftopict-367983-.html
HTH
I refuse to use the term "full" or "half" sheet cake 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" for sheet cake servings:
http://forum.cakecentral.com/cake-decorating-ftopict-302303-feeding.html+80
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