How Big a Cake Do You Need for 20, 30 or 50 People? Work It Out From the Slice
Decide the slice first and the size follows. A 10-inch (25 cm) round feeds 20 people, a 12-inch (30 cm) round feeds 30, and 10- and 12-inch tiers together feed about 50, all based on a medium slice of roughly 4 by 5 cm.
Cut thinner and each cake stretches further. Cut thicker and you run short. The sum below takes a minute, and the three scenarios show how it plays out at real parties.
The sum behind how big a cake you need
Take the top area of the cake, divide it by the area of one slice, then knock off a fifth for end pieces and ragged cuts. A medium party slice measures about 1.5 by 2 inches, which is 3 square inches. The top area is 3.14 times the radius squared.
That gives these counts for round cakes:
- 8 inch (20 cm): about 13 slices
- 10 inch (25 cm): about 21 slices
- 12 inch (30 cm): about 30 slices
- 8- and 12-inch tiers: about 43 slices
- 10 and 12 inch tiers: about 51 slices
Cut slim 1-by-2-inch fingers instead and multiply each figure by one and a half. An 8-inch cake then covers about 20 people.
20 people: the leaving do in a meeting room
Guests stand there balancing paper plates, so most cut small pieces. An 8-inch cake cut into slim fingers covers 20 people. If the boss insists on proper wedges, move up to the 10-inch.
The cake usually appears mid-afternoon, when nobody is hungry, so there is little pressure to go bigger.
30 people: the village hall 60th
Sandwiches and sausage rolls come first, and the cake follows an hour later. A 12-inch round gives exactly 30 medium slices, which leaves no spare. A 12-inch square gives about 38, because the corners add area, and it usually costs only a little more.
If the hall has a long table and guests help themselves, expect a few to take two slices.
50 people: the 18th in a function room
The cake comes out late, after a buffet, and the DJ is already playing. With 10- and 12-inch tiers, you get about 51 medium slices, enough for a firm list of 50 with a slice or two spare. Most parties lose a few guests to illness or traffic, which gives you extra room.
Cut the bottom tier first. Once the crowd thins, you can save the top tier for the next day.
Three questions bakers hear about size
My baker sells in centimetres. Do the numbers change?
No. A 20 cm cake is close to 8 inches, 25 cm to 10 inches and 30 cm to 12 inches. Use the same counts.
How far does a rich fruit cake go?
Further than sponge. Fruit cake is dense, so people cut it in 1-inch squares. An 8-inch round gives around 40 pieces.
Is a square cake always better value?
Only when your guest list sits between two round sizes. If a round fits your numbers neatly, there is no reason to pay for the extra area.
Before you ring the baker
- Count the people who will actually eat cake, not the full invitation list.
- Choose slim fingers or medium slices.
- Run the sum, or use the list above.
- Check the size in the baker’s own units, inches or centimetres.
- Ask how many portions the baker assumes, then compare it with yours.
Back at the village hall, the organiser who did this sum ordered the square cake and had a handful of slices left over for the caretaker. Work out how big a cake you need for 20, 30 or 50 people once, write the number down, and you can quote it to any baker.
