Cake servings are counted by footprint, not by volume, because every slice is the full height of the cake. That makes the arithmetic simple — pan area divided by slice area — and it is why the published charts are what they are.
Two standard slice sizes
The event or wedding serving is one inch by two inches by the height of the cake. It sounds tiny and it is — but a four-inch tall cake makes that a solid eight cubic inches of cake, which is plenty after a meal.
The party serving is two by two inches, twice the footprint. This is what people actually cut at home and what a birthday cake gets divided into.
A nine-inch round is 63.6 square inches, which is 31 event servings or 15 party servings once you drop the part-slice at the end. The published charts say 32 and 16 — the same division, rounded the other way. A slice either way is the size of the disagreement.
Layers multiply, tiers do not
Stacking two layers of the same diameter doubles the height of the slice, not the number of slices. A two-layer nine-inch round still yields 32 event servings — they are just twice as tall.
That is why cake charts quote servings for a standard four-inch tall cake, which is usually two two-inch layers with filling.
A tiered cake is different: each tier is a different diameter and contributes its own count, so the totals are added. The layers field here is for stacking same-size layers into a taller cake.
Round versus square
A square pan of the same nominal size holds noticeably more than a round one — an eight-inch square is 64 square inches against 50 for an eight-inch round, about 27 percent more.
The rule of thumb is that a square pan yields about the same as a round pan two inches larger. An eight-inch square is close to a ten-inch round.
Square and sheet cakes also cut more efficiently, with no wedge-shaped offcuts, which is why caterers prefer them for large numbers.
Be pessimistic about your own cutting
The charts assume a professional cutting with a long knife, wiping between cuts, and taking a consistent inch. Most people cut bigger slices and lose some to crumbs.
Taking 10 to 15 percent off the calculated number for a self-cut cake is realistic. For a cake where people help themselves, take more off.
The exception is a cake that is mostly decoration — a thick layer of fondant or a heavy buttercream shell means fewer people want a full slice.
How much batter
A two-inch deep pan filled two thirds is the target. For a nine-inch round that is roughly five cups of batter; for a nine by thirteen, about eight.
Overfilling is the more common error and it is messy — the batter rises over the rim and the cake domes badly. Underfilling gives a thin, dry layer.
If a recipe makes more batter than the pan should hold, bake the excess as cupcakes rather than forcing it in.