A 9 x 13 is a 1/4 sheet for me.
Wilton says you can get 24 servings out of this....which really you should be able to considering that you usually get 24 cupcakes out of 1 box mix.
but, I've seen so many different variations & different charts, etc., you'll just have to pick one you really like and stick with it. I saw one somewhere that said you can only get 12 servings from a 9x13. Don't know who was eating that cake, but........some people do!
HTH!
This is what I typically go by for sheet cakes
9x13 - 1/4 sheet cake - 24 servings
12x18 - 1/2 sheet cake - 48 servings
18x24 - full sheet cake - 96 servings
I don't have the 11x15 pan, so I can't help ya there.
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