My standard answer ..... I refuse to use the terms "half" or "quarter" sheet cake. Most people don't know what it is either ..... they use "the lingo". If someone asks for pricing for a 1/2 sheet cake, I ask them how many people they need to serve? I get answers anywhere from 15 to 50. They have no idea what they are even ordering!!!
So dont' get hung up on labels. Find out how many people they need to serve, then, as the cake expert, you tell THEM what they need.
Here's a thread in which I've already done the math for servings:
http://forum.cakecentral.com/cake-decorating-ftopict-302303-feeding.html+80
ok i hope someone answers this because i was wondering te same thing
I think it's considered a 3/4 sheet, but I agree with indydebi, most times, people have no earthly idea what they are ordering. They say they want a full sheet, and you tell them the price and how many it feeds, and they quickly say "Oh, I only need to serve 20 people".
Hi - for my cake business, I consider the 12x18 a full sheet - serves up to 54 people. I use 3 cake mixes for it to make it full. I've done two 11x15's together, which make a nice big sheet cake, too (they are normally 1/2 & 1/2). I use 2 cake mixes for the 11x15. I can see the 12x18 being a 3/4 sheet, too.
Hope that helps. I know some places have bigger sheet cakes, etc., but this is what works for me! 
Amy
Is there actually a pan that is 16" x 24"...a full sheet?
Yes there is but you won't fit it in your oven at home. You can piece them together with smaller cakes.
From what I understand most grocery bakeries consider a 16x24 a full sheet.
Where to buy sheet cake pans:
(Don't know if these are good prices, just trying to show what's available.)
http://forum.cakecentral.com/cake-decorating-ftopict-234751-.html
Wilton's cake preparation and serving guides give party size and wedding size yields on layered sheet cakes (both 2 & 3" pans):
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
HTH
Here are the sizes I use (all are single-layer cakes):
1/4 sheet: 9x13x2 ... serves 16-20
1/2 sheet: 12x18x2 ... serves 32-35
Full sheet: two 12x18x2's, side by side ... serves 64-70
When I bake my cakes at home I go by how much batter I use.....not the size of the pan. I use one regular scratch recipe for a quarter sheet cake. I have a 9x13 pan and I have an 8x11. Either one can be a quarter sheet. The 8x11 makes a taller cake so I prefer using it. The amount of cake is the same and the number of servings are the same. With the flatter cake you would just cut the servings a little larger to make up for the height difference. A half sheet for me is double the recipe and a full sheet would be 4x. Same thing if you are using a box mix....1 box=quarter sheet...2 boxes=1/2 sheet....4 boxes=full sheet. The size of the pan really doesn't matter...it is the amount of cake in the pan you are looking for. I agree, you should just quote the number of servings to the customer, not the size of the cake.
so for those of you who serve it as 1/4, 1/2, 3/4, & full sheets... what do you use to make a 3/4?
at the bakery i work at they bake full sheets 16x24, and from there cut 1/4 sheets (8x12)
1/3rd sheets (8x16)
1/2 sheets (12x16)
3/4 sheets (16x16)
thats commercial sizes. for myself i too always ask how many they want to serve and advise from there.
i do 9x13, 11x15, 12x18 and 2 - 11x15 to serve 70 and 2 -12x18 to serve 100
Wow, now I'm really confused. . .I thought 9 x 13 was 1/4, 11 x 15 was 1/3, and 12 x 18 was 1/2. If you lay out 4 - 9 x 13 pans the measure would be 18 x 26 which is very close to 2 - 12 x 18 pans (18 x 24), which is why i thought that 12 x 18 was half sheet.
Wow, now I'm really confused. . .I thought 9 x 13 was 1/4, 11 x 15 was 1/3, and 12 x 18 was 1/2. If you lay out 4 - 9 x 13 pans the measure would be 18 x 26 which is very close to 2 - 12 x 18 pans (18 x 24), which is why i thought that 12 x 18 was half sheet.
I agree with jonicca, that's the way I've always thought the 1/4, 1/2 and full sheet went. But mainly I don't use that. I always ask how many servings they need and go from there. If they cut in rows of 6 X 4 on a 12 X 18 they get 54 servings. So I judge a 1/4 sheet to serv 25, 1/2 sheet to serv 50 and a full sheet to serve 100. (I give a little in the servings because not everyone knows how to cut 6 X 4 or in even 1.5 X 2" pieces!)
If I'm ever at the same party, I always get asked to cut the cake!
(I give a little in the servings because not everyone knows how to cut 6 X 4 or in even 1.5 X 2" pieces!)
I try to put something in the design of the border (a small dot, for example) to show where 2x2 cuts would be made.
(I give a little in the servings because not everyone knows how to cut 6 X 4 or in even 1.5 X 2" pieces!)
I try to put something in the design of the border (a small dot, for example) to show where 2x2 cuts would be made.
Are you in my kitchen Indydebi?? I have done this on the last few cakes I did and no one even notices until I point it out to them. The christmas presents in my photos all have dots of pearl dust on the bottom border which are actually the cutting guide... Did another one this weekend where I put red dots on the white border and everyone just thought it was part of the design... Muahahaha little do they know!
I try to put something in the design of the border (a small dot, for example) to show where 2x2 cuts would be made.
What a good idea Why don't these thing occur to me? 
I guess it's a matter of experience... Thanks!
i didnt read through all the posts, but 12 x 18 is a half sheet cake and the 11 x 15 would be 1/3 sheetcake. 18 x 24 would be full.
How I do it...
1/8 sheet (7x11)
1/4 sheet (9x13)
1/3 sheet (11x15)
1/2 sheet (12x1
full (18x24....two 12x18s put together)
Agree with Indydebi that the average person doesn't know exactly what they are ordering. All they do know is it's a sheet cake...after that, they throw out the first word they think of...."uh, 'half' sheet, yeah that's what I want"
It's best to ask them the # of people they are serving, then tell them their options.
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