Cake size calculator
One serving per guest. Enter how many people will eat cake and pick a cake, cupcakes or both.
Festidy's plan counts cake servings with your RSVPs, so the order matches who is coming.
Make my party planServings by pan size
| Pan | Cuts into (2 x 2 inch slices) | Festidy plans it for |
|---|---|---|
| 6-inch round | 12 | The small cake for candles |
| 8-inch round | 20 | Up to 12 guests |
| 10-inch round | 28 | Not used: pick a quarter sheet |
| Quarter sheet (9 x 13 inch) | 24 | 13 to 24 guests |
| Half sheet (12 x 18 inch) | 54 | 25 to 48 guests |
| Cupcakes (a dozen) | 12 | Up to 12 guests, then one more dozen for each 12 |
Slice counts are from a baking site's cake serving chart, checked October 21, 2026. Its quarter and half sheet counts (24 and 54) match a second chart for the same 2 x 2 inch slice. Bakeries cut differently, so ask yours.
How we calculate
- Cake: one serving per guest, kids, grown-ups and siblings included. Up to 12 servings is an 8-inch round or a dozen cupcakes, up to 24 a quarter sheet, up to 48 a half sheet, and more than that is one half sheet per 48.
- Why our limits are lower than the chart: the chart counts the most a pan gives when cut small, a 2 x 2 inch slice. We plan a real slice each, plus the corner piece everyone wants. The quarter sheet limit (24) matches the chart, the half sheet limit (48) sits under it (54), and the 8-inch round limit (12) is well under it (20).
- Cupcakes: guests rounded up to the next dozen. The rounding leaves a few spares; if your count is an exact dozen, add a half dozen for seconds and drops.
- Both: every guest gets a cupcake, and a 6-inch round is the small cake for the candles and the photos.
- Prices (rough, and they change by town): a decorated grocery bakery cake is about $20 to $45, Festidy's planning range, and we double it for a half sheet, which is about twice a quarter sheet. A custom bakery cake for 20 is about $100 to $150, so we use about $5 to $7.50 a serving, and never less than the 12-guest price. For cupcakes we guess about $15 to $25 a dozen at a grocery bakery and about $3 to $5 each at a custom bakery.
- Order ahead (typical, not a promise): a grocery bakery about 2 to 3 days ahead, a custom bakery 2 to 3 weeks ahead. Busy seasons book earlier, so call and ask.
Last updated October 21, 2026. Free, no sign-up.
New to Festidy? See how the free birthday party planner works.
Tips for ordering a party cake
- Count everyone who might eat it. Siblings, grandparents and the kid who "just wants a corner piece" all count.
- Choose cupcakes when you do not want to cut. They hand out in seconds and nobody waits at the table.
- Tell the bakery the number of guests, not just the size. They know their own slice size and will correct you.
- Candles: one per year plus one to grow on is a tradition. For a 6th birthday, that is 7. Never leave lit candles unattended.
- Ask about allergies on the invitation. If a guest cannot have the cake, a few safe cupcakes keep them in the party.
Questions
What size cake feeds 20 people?
A quarter sheet cake (9 x 13 inch), which serves up to 24 in our plan, or 2 dozen cupcakes. An 8-inch round is a little small: we plan it for 12.
How many people does a half sheet cake feed?
Up to 48 in our plan, so it suits 25 to 48 guests. A baking chart gets about 54 from 2 x 2 inch slices, and the gap leaves room for bigger pieces.
How many cupcakes do I need for 30 guests?
3 dozen, which is 36: one per guest and 6 spares. Bakeries sell by the dozen, and spares cover seconds and drops.
How many does a quarter sheet cake serve?
Up to 24 in our plan, which matches the chart's 24 slices of 2 x 2 inches.
How far ahead should I order a birthday cake?
Typically about 2 to 3 days for a grocery bakery and 2 to 3 weeks for a custom bakery. That is a common pattern, not a promise, so call yours.
How much does a birthday cake cost?
About $20 to $45 for a decorated grocery bakery cake. A custom bakery cake for 20 is about $100 to $150. Prices change by town.