I need to make a cake between 70 and 90 people and I have two half sheet pans and I was wondering well that be enough

What are you calling a half sheet? 12x18 or 11x15?

http://www.aztecrental.com/cakeserve.htm

http://www.publix.com/custom/decoratedcakes/CakeInformation.do

Here are a couple of links with different serving size guides.. I'm not sure what size you're using for a half sheet cake.

hi i use the wilton 11x15 as my half sheet so this is for that size cake.but if you have any wilto year books they have a serving fuide and cutiing guide for every size pan they have (excluding character pans) anyway:

an 11x 15 usually serves around 60 if you cut in 1x2 in slices. so you should be able to get more than enough out of your full sheet. i cut mine larger than wilton says so i usually only get 40 -50 slices from my half sheets. i hope this helps

I work in a bakery, and our full sheet serves 77-if you serve 2x2 pieces. Hope that helps you-Carol