How Much Birthday Cake Do I Need?
Work from servings required → intended slice size → actual cake yield. Working from diameter alone can create false precision.
Step 1: estimate cake eaters
Start with the guests likely to be present for cake. Add a reserve if you want spare servings or expect uncertain attendance.
Step 2: decide how cake is being served
A small slice after a full meal is different from a generous dessert portion. Tall cakes and layered cakes can also be cut differently from shallow home-baked cakes.
Step 3: use the exact cake's serving guide
Bakeries often publish serving counts and cutting guides for their own products. Those are more useful than assuming every cake with the same diameter has the same height and yield.
Step 4: round up only when needed
If you need 28 servings and the chosen cake yields 30 using the intended cut, one cake covers the plan. If it yields 14, the calculator will show that two are needed.