What this calculator does
Cake servings come from area rather than diameter. A round cake area is πr², so a 25 cm cake gives about 38 servings at a 13 cm² serving size while a 20 cm cake gives 24.
The square relationship is what surprises people. Going up one size from 20 cm to 25 cm adds only 5 cm of width but more than half again as many servings, because the area rises from 314 to 491 cm². Ordering the next size up is usually a far bigger step than it appears.
The formula
The cake area is calculated from the shape: π times the radius squared for round, or length times width for rectangular. That area is divided by the serving size to give the number of servings. The calculation treats the cake as a single layer of area, so a multi-layer cake of the same footprint gives the same serving count with taller slices.
| Term | Meaning |
|---|---|
| Serving size | The area of one slice, conventionally around 13 cm² for a party serving and smaller for formal wedding portions. |
| Party serving | A generous slice, typically around 2.5 by 5 cm in footprint. |
| Wedding serving | A smaller formal portion, often around 2.5 by 2.5 cm, giving roughly twice as many servings from the same cake. |
The inputs explained
| Field | What to enter |
|---|---|
| Shape | Round or rectangular, which determines how the area is calculated. |
| Diameter (if round) (cm) | Diameter in centimetres, for a round cake. |
| Length (if rectangular) (cm) | Length in centimetres, for a rectangular cake. |
| Width (if rectangular) (cm) | Width in centimetres, for a rectangular cake. |
| Serving size (cm²) | Serving size in square centimetres. Around 13 for a party slice, lower for formal portions. |
When to use it
Sizing a cake for a guest count
Working backwards from the number of guests to the cake size needed, which is the usual direction the question runs in.
Comparing two cake sizes
The area difference between adjacent sizes is much larger than the diameter difference suggests, and seeing both makes the choice clearer.
Pricing a cake per serving
Per-serving pricing needs a defensible serving count, and area divided by serving size is the standard basis.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How many servings does each cake size give?
A range of round cake diameters at a standard party serving size.
Questions
How many people does a 25 cm cake feed?
About 38 at a 13 cm² party serving, or roughly twice that at formal wedding portions. The serving size assumption matters as much as the cake size, which is why cake charts from different sources disagree so widely.
Why do wedding cakes serve so many more?
Because wedding portions are much smaller, conventionally around 2.5 by 2.5 cm against a party slice roughly twice that footprint. The same cake can therefore be quoted at two quite different serving counts, both honestly.
Does cake height affect the serving count?
Not in this calculation, which works from area alone. In practice a taller cake gives more substantial slices of the same footprint, so a very tall cake might reasonably be cut into smaller pieces. Conventional serving charts assume a standard height of around 10 cm.
How do round and square cakes compare?
A square cake of the same width gives noticeably more, since its area is the width squared against π times the radius squared, which is about 27% more for the same span. Square cakes are also simpler to cut into even portions.
For converting a recipe between pan sizes, see the cake pan converter. For pricing the finished cake, see the cake pricing calculator.