I am going to make the cakes for our company party and I need enough to feed 150. I plan on using the 12x18 and making them 2 layers and having the venue cut the pieces 2x2". Will someone help me please? Thank you in advance.
evelyn
In your case it is simple math:
12 x 18 pan means 6 x 9 pieces in the size you want (54 servings). You need 150, meaning you need 3 completed cakes, getting 162 servings. And you need to bake twice as much - the two layers - also you have to bake 6 sheet cakes.
Confirming what majk_ze posted....
12x18x4=864 cubic inches
2x2x4=16 cubic inches (Wow, OP, you're serving HUGE pieces of cake!)
854/16=54 servings
Each 4" high (2-layer) 12x18 cake yields 54 2x2x4 servings, so if you need 150 servings, you will need 3 cakes.
Thanks to everyone that has answered 
Would I be better off making the pieces 1" by 2" then? We are having a sit down dinner and since there will be about 175 people there I know not everyone will want cake that is why I opted for the 150 count. Sorry for so many questions but normally my cakes for this many people are wedding cakes. Thanks again for all the help!!
evelyn
I would go with the 1" x 2" slice - more then enough for me and my family. There isn't single person who would eat a 2"x2"x3-4" slice and not have any cake left on the plate.
If you have your heart set on using the 12" x 18" cake pan, this could go either - one 12" x 18" cake (108 servings) and one 12" x 9" cake (the layer cut in half and stacked; 54 servings) - the same total count of 162 servings.
Or you could bake two 2layer 19"x18" cakes and cut them in 1 1/3" x 2" pieces - not a standard serving size, still more then enough IMHO, you will still get cake left on plates. For this, cut each cake in 9x9 servings from one cake (81 servings), with total count again 162 servings.
In this case, saying each cake needs to be cut in 9 rows and 9 columns is easier then give a size.
I would go with the 1" x 2" serving size and 162 servings. You have only about 13 people "planned" who won't get cake. In this numbers, I suppose there will be more people who don't want cake and this you can use for the "greedy ones" who would like seconds.
If you're having a sit down dinner, then I would think you would want to treat it as a wedding and serve 1"x2"x4" slices.
In the past, when we've made cake for our own personal use, we used to do one layer torted. It wasn't until we started making cakes in a more professional manner that we started stacking. You don't realize until you've made a 4" tall cake how much cake is really there. That's because we're so used to cutting it from the other direction (across the cake) to get a size that's "decent" for eating. It's just a matter of shifting perspective.
Paul
People hear 'one inch' and they think 'paper thin'. It's not. It's plenty of cake, especially after dinner. I think the reason so many people dont' order dessert after dinner is that we've super sized everything to the point that we CAN'T eat a whole mean (meat, veggie, salad, dessert).
Hubby and I found a restaurant lately and when we first ate there, our initial thought was "wow, these are tiny servings." But when we were done with dinner, we realized we were full. Not stuffed, not bloated, but a nice comfy full feeling .... like you're SUPPOSE to feel after dinner.
Go with the 1x2x4.
Thanks everyone for the help I think I will cut the sizes so I can actually see what they look like and I know that some will not want cake anyway. I knew I could count on getting help here 
evelyn
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