Here is the measurements and servings I use, BUT everyone is different, unfortunately, there is no industry standard for sheet cake sizes!
Sheet.......................Servings (2x2x3)
Quarter (9 x 13).....29
Half (12 x 18).........54
Full (16 x 24)..........96
Definitely call to your local bakeries and see what your local standard is.
Some places a 12"x18" is a 3/4 sheet while others it is a half sheet, and even other places consider it a full sheet.
Personally I never use the word "sheet cake" when talking to a customer. I find out how many people they want to serve then tell them the options they have for the cake size and shape to get close to their servings.
I agree .... no labeling "half" or "quarter". Below is a link on this subject, plus inside that one is a link to where I've done the math on sheet cakes.
http://forum.cakecentral.com/modules.php?name=Forums&file=viewtopic&p=3245473#3245473
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