I am confuse, okay in the wilton book a 11x15 pan double layer serves 60 only so wouldn't a singe layer only serve 30 and a 12x18 double layer would only serve 72 so wouldn't a single layer be half of that . Most of my birthday sheet cakes are singe layer s, I am wondering where you are getting your serving sizes from or what I am doing wrong
The Wilton cutting guides state that a party serving is 1-1/2 x 2 x 4" high.
If you only want to serve a single layer and get half the servings - the pieces would need to be 1-1/2 x 2 x 2" high.
Most cut single layer sheet cakes into 2 x 2 x 2" pieces which yields a different number of servings than Wilton.
HTH
http://forum.cakecentral.com/cake-decorating-ftopict-302303-feeding.html+80
Above is a link to a thread where I've posted how to do the math on sheet cakes.
Indebyi
If you could only do a chart like the wilton chart on the 2x2 serving sizes . Yet i see what you are doing now will i be correct in saying that a
9x13 single layer serves 24 two side by side would serve 48
11x15 single layer serves 35 two sides by side would serve 70
12x18 single layer serves 54 and two side by side would serve 108
another sheet cake question
would you yeild more serving if the cake was a double layer or single layer consider you are cutting it the same way
The servings (yield) would remain the same whether it was a single or layer cake if the serving sizes remained the same.
Usually, in a layer cake the servings sizes are smaller than a single layer sheet cake because there are, well - two layers not one.
How many servings are you trying to get from a single layer sheet cake? (Then Debi could figure out what size the serving should be.) 
Sorry Debi, math isn't my strong point. 
another sheet cake question
would you yeild more serving if the cake was a double layer or single layer consider you are cutting it the same way
If you cut them the same size but put double the cake on top of it, then you will get the same number of pieces, only they will be twice as tall.
That's why in the post (see link above) the double-layer cakes are 1x2 instead of 2x2......they are twice as tall therefore 1/2 as wide.
*IF* you don't have a base price per serving which is determined by a specific size of cake (i.e. 2x2 for sheets and 1x2x4 for layered), then be SURE you charge for double cake on the double layers!
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JanH, you are too funny! And I'm very comfortable being a numbers geek! 
"Debi Does Data!"
I tell my customers that I consider a sheet cake serving to be 2" x 3" or a little smaller than a business card. It's a fairly substantial serving size for normal family type gatherings, at least my family.
1/4 sheet 9x13 = 19 servings
11x15 = 27 servings
1/2 sheet 12x18 = 35 servings
full sheet 16x24 = 64 servings
va cake lady
thats going down in serving, I look at a flyer walmart had and there serving size yeild more for their sheet cake then mine were, yet mine seem to be a bigger cake that's what made me post the question. If my prices are higher , I just wanted to give the correct serving sizes.
I'm so glad this was posted, I don't know why what seems to be such a simple subject is so difficult for me but I'm baffled by the serving size too this is great info guys.
Not only am I new in this forum but bad with numbers and things as well and now i must still divert your numbers into my "currency". Initially i thought 2x2 is small and then I realise that Your are talking inches and i'm thinking cm's which would then be 4 x 4 and then it is pretty an okay size cake 
So hold on to me, i'll catch up with you guys soon - maybe a bit slower but i'll get there soon. 
I've been very confused about the Sheet Cakes and whether they are Single Layer or Double Layer? From what Debi said the 2x2 is a Single Layer Cake and the 1x2 is the Double Layer Cake? Do I understand that correctly?
I need the 12x18 to meet the 108 servings. So I'm assuming if I make it a Double Layer sheet cake and cut it 1x2x4, then I should get my servings?
Also, typically, are Sheet Cakes Single Layer? I never really knew.
Thanks!
Wendy
I've been very confused about the Sheet Cakes and whether they are Single Layer or Double Layer? From what Debi said the 2x2 is a Single Layer Cake and the 1x2 is the Double Layer Cake? Do I understand that correctly?
I need the 12x18 to meet the 108 servings. So I'm assuming if I make it a Double Layer sheet cake and cut it 1x2x4, then I should get my servings?
Also, typically, are Sheet Cakes Single Layer? I never really knew.
Either... sheets are typically single layer, if double layer -- they're often called kitchen cakes. Yes, you cut 2" tall cakes into 2" x 2" squares, and 4" tall cakes into 1" x 2" servings to achieve industry standard 8 cu. in. servings.
Yes, you'll get 108 servings from a 12x18 if you cut 12 rows by 9 columns.
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
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