There seem to be two commonly used charts: Wilton's and Earlene's. Wilton says that a two layer 9" cake serves 32. Earlene's Cakes says a 9" cake serves 22. Straightforward, if contradictory.
However, both of these amounts are using the industry standard serving size of 1" × 2". That is a piece of cake the size of the business end of a fork.
|Picture from Wikipedia|
For those of use who want to eat cake, there is something called the Party Serving. We're getting a little better here.
But how do I cut my son's birthday cake? And what if I would rather cut wedges than concentric circles? What about home bakers and amateur party party planners, for whom the industry standard isn't relevant, but appetites are?
Well, I did some research, did a lot of math, and cut some cakes. I drew from Lark Cake Serving Guide, which I found to be the most clear.
According to Lark Bakery, 10" is the largest round cake that can be cut into wedges. That's the largest size on my cake serving chart.
I used two differently sized slices: generous and sensible. My sensible portion is about the size of a cupcake, and my generous portion is, well, generous. As Wilton does, I assumed that the cake is two layers deep. The cake above is cut into generous portions. Specifically, a generous portion is approximately 6.5 square inches, with a piece 2 by 3½-inches. A sensible portion is 3.5 square inches, with a piece 1¾ by 2-inches.
|Round Cake Cutting Diagram|
For smaller square and rectangular cakes, it is convenient to think of how to divide a cake along its edge—in thirds, fourths, or sixths. I have listed how to cut the slices on each edge for all sizes but 13 by 18-inch, which is in my experience too large to cut that way.
Square and Rectangular Cake Servings
|9 by 13-inch||18||6||3||32||8||4|
|13 by 18-inch||36||—||—||64||—||—|
11/12/2012—updated to clarify picture source