I'm giving this a bump. If I were home I would look it up in the Wilton yearbook. They have a table in the middle of the book for all size cakes of how much batter is needed as well as how much icing is needed. If anyone looks at this and has a book handy - maybe you could reply? Thanks. 
The Wilton book says that 8 cups icing will ice and fill a 2 layer 11X15.
I always seem to need more. 
Wow, 8 cups is less than three batches. I always use at least two batches, which is 6 cups, to ice and decorate anything 9 x 13 ........... seems like it would need more than three batches...........
Then again, I've been told there's plenty of icing on my cakes..... that's one thing I'm working on, is cutting the amount of icing I use.
For future reference, Wilton has this listed on their site:
http://www.wilton.com/cake/cakeprep/baking/times/party_2inch.cfm
It gives not only approximate cups of icing needed, but also helpful batter amounts and baking times for different pans.
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