http://www.wilton.com/cakes/making-cakes/baking-party-cake-2-inch-pans.cfm
That has a list of pan sizes and their serving amounts listed as well as some other useful information. Good luck 
Hi, I know lot of members use this chart. HOpe it helps you as well.
http://www.earlenescakes.com/ckserchart.htm
Use the Wilton chart. It is right on for serving sizes.
It is very easy to figure # of servings in sq cakes. As an example if the cake is 3 or 4" tall, it is cut into 1x2x4 pieces. So a 12 sq would be 12x6=72 servings.
If it is only 2" high (a single layer either split & filled or not) it would be served in 2x2x2 pieces so that 12" sq will give you 36 servings.
That was the same question I had for a baby shower, around 100 people... would 2 9x13 cake put together be enough... I checked the chart and It should be???? thanks
Let's walk thru the math.....
two 9x13's = one 18x13.
Single layer, cut in 2x2x2" pieces, means the cake will be cut in 9 columns (18 divided by 2") by 6 rows (13 divided by 2" ... rounded) = 54 pieces. You need to bake two 9x13's for this cake.
Double layer, cut in 1x2x4" pieces, means the cake will be cut in 9 columns by 13 rows (oh, let's use 12 to allow for any cake shrinkage!) ... 9 columns by 12 rows = 108 pieces. You need to bake four 9x13's for this cake.
ljhow623, also remember that it is very rare that you will be able to come up with exactly the number of servings the client is needing. They need to decide if they want a little less or a little more.
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