How this calculator works
A single-layer cake is cut into portions by area, so the number of servings is the area of the cake top divided by the area of one slice. A round pan of diameter d has an area of pi x d squared / 4, and a square pan has an area of side squared. Wedding slices are small, about 1 by 2 inches; party slices are about 2 by 2 inches; dessert portions are larger. Multi-tier cakes add the areas of each tier, and this calculator models one tier at a time.
Formula
round pan area = pi x diameter^2 / 4. square pan area = side^2. servings = floor(pan area / slice area). For a guest count, required area = guests x slice area, the matching round diameter is 2 x square root(area / pi), and the matching square side is square root(area). One inch is 2.54 cm.
Worked example
An 8 in round pan has an area of 3.14 x 8^2 / 4 = 50.27 square inches. At the party slice of 4 square inches, that is floor(50.27 / 4) = 12 servings. For 30 guests at the same slice size you need 120 square inches, which is a round pan of diameter 2 x square root(120 / 3.14) = 12.4 inches, or a square pan of side 10.95 inches.
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Frequently asked questions
Why do cake charts give a range of servings?
Because cutting is not perfectly regular and slice sizes vary between events. A chart may allow 20 to 24 servings for an 8 in round cake at party size. The calculator floors the division, so it gives the conservative end of that range.
How do tiered cakes change the count?
Each tier contributes its own surface area. A three-tier cake with 10, 8 and 6 inch rounds has 78.5 + 50.3 + 28.3 = 157.1 square inches of top area, giving 39 party slices. The sides of each tier are not normally counted for servings.
Should I use the tin size or the baked cake size?
Use the inside diameter of the pan. Cakes shrink slightly as they cool and are often trimmed, so the usable surface is a little smaller than the tin. For a large event, round the guest count up rather than down.
Does cake height matter?
Height changes the portion volume, and tall layer cakes are often cut into thinner slices. The area method still gives the number of top-surface pieces. For a two-layer cake, a 1 x 2 inch slice contains two layers and is a generous wedding portion.