I have a lady that wants me to do a tier cake that will feed 30-40. Can anyone tell me what size cake i need to make to feed that many people? Thank you for any help.

Ahttp://www.google.com/url?sa=t&source=web&cd=1&ved=0CC0QFjAA&url=http%3A%2F%2Fwww.wilton.com%2Fwedding%2Fwedding-cakes%2Fwedding-cake-data.cfm&ei=_KCyUPeZO8GayAGboYGYCA&usg=AFQjCNE7yRZtGNwE0KEmGIr9LxydLETqwg&sig2=7EpTpThCFvLqZ9Fh47wdSA

ASo by that chart it would be a very small cake 8 inch with a 6 inch on top of it.....just make sure they understand how small the slices are actually going to be. Some people most cut very llarge slices so they would be expecting a very large cake

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

The above chart is based on a piece which is 1x2x4" and believe it or not, is not a small piece.  This is the same standard size that is used in the industry,  So, if after you tell the customer that the 6/8 serves 36  based on this standard size ,and the customer wants to serve bigger pieces, be sure you are charging appropriately for a bigger cake.