You want the surface area of the pan to be approximately equal to that in your recipe, so that the depth of cake (and the cooking time) are around the same.

Two 9" diameter pans have a surface area of:

2x [(4.5 x 4.5) x 3.142]=127.17 sq in

One 9 x 13 = 117 sq in

That is equal enough for a fair try in my book; just do a bit of fork testing toward the end of the process to make sure the cake is done.

The math is pretty easy, see? I'm sure you can work out the number of servings from here.

well, the recipe for Pillsbury cake mix says the same time and temp for two 9" rounds or one 9"x13" sheet, so I would use the same time and temp listed on your recipe for the 9" pans.

http://www.pillsburybaking.com/recipes/details/1851

As for how many it will serve... that depends on how small the servings are.

You could also do cupcakes instead. More time in the kitchen, but easier to serve, IMHO.

If you didn't think of it already, I would look around for a sheet cake recipe. Baking proportions are tough and if you're off in the slightest, the cake could change dramatically.

Also, how many pieces will we get out of a 9 x 13 cake? I am expecting about 10 kids and 12 parents. Or should I make two cakes? Thanks!

I am making my son's birthday cake. I have made this recipe for a yellow cake before in two 8 inch round pans. Recipe says you can also use two 9 inch round pans. I want to make a sheet cake, I guess in a 9 x 13 pan. Can anyone tell me how much batter I need to make? One and half times the recipe? Two times the recipe? I could google volume and circumfrence and terms like that to try and figure it out, but it's probably been 20 years since I tried to do any kind of math like that. Also, I'm assumning the baking time will be just a little bit longer, but otherwise it should turn out pretty well, right?

Thanks!