Pizza ordering is the rare catering problem with a genuinely simple answer, which is why it's maddening that groups still get it wrong. The whole thing is the 3/8 rule: three slices per adult, eight slices per large pizza. Everything else is adjustment.
The lookup tables
All adults, pizza is the meal (large, 8 slices)
| People | Slices | Large pizzas |
|---|---|---|
| 5 | 15 | 2 |
| 10 | 30 | 4 |
| 15 | 45 | 6 |
| 20 | 60 | 8 |
| 30 | 90 | 12 |
| 50 | 150 | 19 |
| 100 | 300 | 38 |
Mixed crowd (kids average 2 slices)
| Group | Slices | Large pizzas |
|---|---|---|
| 10 adults + 5 kids | 40 | 5 |
| 15 adults + 10 kids | 65 | 9 |
| 20 adults + 15 kids | 90 | 12 |
The adjustments
- Other food on the table: drop to 2–2.5 slices per adult (−15%).
- Teenagers, sports teams, anyone post-hike: plan 4 per person (+20%). Teenagers are adults for pizza purposes, and then some.
- Late-evening order (after drinks): +1 slice per adult. This is empirically robust.
- Thin crust / small local slices: some independents cut larges into 6; ask, because the 3/8 rule silently becomes a 3/6 rule and your order is 33% short.
Topping strategy for groups
The reliable split for a crowd that didn't pre-vote: 40% single-meat classics (pepperoni), 30% cheese/margherita, 20% vegetarian, 10% adventurous. Two rules survive all group-size math: never skip plain cheese (it's everyone's second choice, which makes it the safety stock), and never let one person's favourite exotic combination occupy more than one pizza. If dietary needs are in play, one dedicated gluten-free or vegan pizza per 2–3 affected guests, clearly labelled and physically separated, or it will be innocently eaten by everyone else within minutes.
Value math
Pizza area scales with the diameter squared. A 14″ large is 154 sq in; a 12″ medium is 113. The large has 36% more pizza and typically costs 15–20% more — larges are nearly always the right unit, XLs where offered are better still. Two mediums ≠ one large plus change; it's the other way around.
The pizza calculator runs all of this — kids, hunger level, slice counts — and suggests the topping split for your exact order.