Not a guess at "how many guests", but a real weight, diameter and portion count — with cake height and cutting style, because those move the result the most.
+17% portions from the same weight (guide gives 15–20%).
Default 10 cm. The now-popular 15 cm is half as much weight and portions again at the same diameter.
| Diameter | Weight | Guests |
|---|---|---|
| ⌀16 cm | 1,21 kg | 10 |
| ⌀18 cm | 1,53 kg | 13 |
| ⌀20 cm | 1,88 kg | 17 |
| ⌀22 cm | 2,28 kg | 20 |
| ⌀24 cm | 2,71 kg | 24 |
| ⌀26 cm | 3,19 kg | 28 |
| ⌀28 cm | 3,69 kg | 33 |
| ⌀30 cm | 4,24 kg | 38 |
| ⌀32 cm | 4,83 kg | 43 |
A sponge cake layered with buttercream weighs roughly the same per cubic centimeter regardless of diameter — that lets weight be calculated from geometry alone: base area times height times density. A mousse or meringue cake weighs differently and needs its own table.
Two cakes of the same diameter but different height are a completely different number of portions — height enters the formula linearly, so 15 cm instead of 10 cm is immediately +50% weight and portions. Customers rarely ask about this, and it's one of the most common reasons for underpricing.
Cutting in the kitchen onto plates yields more, more even portions from the same weight than cutting into wedges at the table — the guide gives a 15–20% difference. Worth accounting for on large wedding cakes.
At the standard 110 g portion, about 9 people. At the 90 g wedding portion, about 11 people.
At a 110 g portion and 10 cm height that's theoretically about 30.6 cm — in practice you'd order the nearest real ring size, ⌀32.
At 10 cm height that's theoretically about 20.6 cm, rounded up in practice to the real ring size ⌀22. At 15 cm height the same kilogram of weight gives a smaller, about ⌀18 cm diameter.
Calculate the cake's weight (base area times height times density) and divide by the weight of one portion. The calculator above does this automatically for both directions.