After browsing the web trying to find "standard" cake sizes I am more confused than ever!
Can anyone here help me figure out some sort of standard for rectangle cakes? (sheet pans?)

I have a pair of basic rectangle pans which are 9x12 and my understanding is that this is considered a 1/4 sheet size.......
I baked a cake for 100 ppl this weekend and it was so complicated because I made 3 of these 9x12...(two layers so it was a total of 6 cakes....if that makes sense?) And basically I ended up with a 12x24x3 rectangle.

Customer seemed pleased but I want to buy a 1/2 sheet pan in order to reduce my baking time.....but this is where my confusion begins!

Turns out a 1/2 sheet is largely variable! 11x15 or 12x16 or 12x18 or 13x18

so which is "standard"? Then it gets more confusing when I try to calculate my brain's logic that a full sheet should be two 1/2 sheets....right?

Well, turns out a full sheet is just as variable! 16x24 or 14x22 or 18x24 or 18x26?

So, I guess this discards the 11x15 as being a "true" half sheet since a full sheet is not 15x22??

Augh! My brain!.....please can someone tell my what size pan I should get?

I appreciate any input!

AH the great debate.........my first 1/2 sheet pan was a 12"x18" and I used doctored cake mixes and it took me 3 batches of batter.......I soon figured out (when trying to compete with pricing) that an 11"x15" was only a LITTLE bit smaller in appearance but used only 2 batches of batter, so as soon as I had the $$$ I bought an 11"x15" and that's what I continue to use. I call that a half sheet cake and now call my 12"x18" a 3/4 sheet cake.
2 of the 11"x15" is what I consider a "full sheet cake", 2 of the 12"x18" is considered an X large sheet cake and is a pain since I have to cut off some of each of the cakes and sandwich the pieces in the center because commercial "full sheet" cake boards aren't big enough to hold them!

Soooooooooooooooooooooooooooo, my advice is use what works for you...just be sure to tell people how many servings they can get. One person's full sheet is not anothers!

http://cakecentral.com/cake-decorating-ftopict-688804-sizes.html

Check out Debi's response on this subject.

I'm with her, don't worry about sizes, go by servings.

For sizes I would go by this:

http://en.wikipedia.org/wiki/Sheet_pan

which is the same as this:

http://www.pastrychef.com/SHEETPANS_p_1271.html

I'm not sure though, how these compare to the what the grocery stores sell.

I am assuming you're in the US.

The Wilton chart can help you with the industry standard:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

I wrote this in another thread about full sheets:

A commercial Bun pan is 18" x 26" (outside measurement), and because they are tapered for nesting or making them stackable, the inside measurement is 16.5" x 24.5".

A commercial full sheet is 16" x 24". They are baked in 16" x 24" bakeable cardboard trays that fit into the Bun pans (flat surface portion) which are used during baking for support and handling purposes.

A true commercial full sheet (16" x 24") serves 96 (unit wt. 106-124 oz.).

If this size pan doesn't fit in your oven, and you are baking two 12" x 18" (54 serving) half sheets, they would serve 108 total.

FYI, there's also extenders for these sheet cakes, to minimize your baking time.

http://www.pastrychef.com/SHEETPAN-EXTENDERS_p_1270.html

I bought this set a while back after reading a post on CC about full sheet, half sheet, quarter sheet... http://www.globalsugarart.com/product.php?id=21102&name=Rectangle%20Cake%20Pan%20Set%203%20inch%20Deep%20by%20Fat%20Daddio's

First of all thank you to everyone for your input and help!
KHalstead, your info was extremely helpful! After reading this, I think I will probably go ahead and purchase the 11x15 to start and if I am needing larger size THEN I can go ahead and get the 12x18.
This brings me to another question though......how many servings do you calculate for the 11x15? (I usually don't make one layer cakes.....they are always two layers and filled).
I guess I should mention that I live in Mexico.....so grocery sizes may be a little different, but I am sure I can adapt!

i always go by servings as well, since that's how the customer orders anyways. i don't make "sheetcakes" per se, but if i'm making a book shaped cake, football field, shoebox, etc. then obviously i'm using them to make the rectangles. they're always at least two layers of cake with a filling in b/t and at least 4 inches tall. in this case, i use the wilton standard servings sizes to calculate the # of servings. it's 1inchx2inchesxheight (min. 4 inches). so the math is quite simple to figure out! if you are dealing with a 10x6 rectangle (just to keep the math easy), then do 10 (for the 1 inch) x 3 (6/2... for the 2 inches) = 30 servings. if you're doing a true 11x15 pan (we'll round here), then say 11 x 7 (i error on the smaller # side) and you'll get 77 servings. if it's a 9x13, then do 9x6 and you get 54. HTH!