Ask a caterer how much alcohol to buy and they won't reach for anything mystical. Nearly the whole industry plans from one rule: two drinks per guest in the first hour, one per guest for every hour after. It isn't a claim about any individual — it's an average that has survived decades of real parties, where the two-glasses-all-night people and the enthusiastic uncles cancel each other out.
The formula
Total drinks = drinking guests × (2 + (party hours − 1))
So a 4-hour party plans 5 drinks per drinking guest. Note the word drinking: count only guests who'll actually drink alcohol. In most Western crowds today that's 65–75% of adults — sober guests, drivers, pregnant guests and people who just don't feel like it add up faster than hosts expect (the non-alcoholic guide covers what to buy them instead).
Converting drinks to bottles
| Container | Servings | The math |
|---|---|---|
| Beer can/bottle (355 ml) | 1 | one serve, done |
| Wine bottle (750 ml) | 5 | 150 ml standard glass |
| Spirits bottle (750 ml) | ~17 | 44 ml (1.5 oz) shots |
| Spirits bottle (700 ml, UK/EU/AU) | ~15 | same shot size |
| Case of beer | 24 | — |
Worked examples
20 guests, 4 hours, beer and wine only
Say 15 drink alcohol. 15 × 5 = 75 drinks. Split 55/45 beer/wine: 41 beers (two cases covers it) and 34 glasses of wine = 7 bottles. Total spend at ordinary retail prices: roughly $120–160.
50 guests, 4 hours, full bar
Say 38 drink. 38 × 5 = 190 drinks. A 40/30/30 split gives 76 beers (3 cases + 4), 57 wine glasses (12 bottles), 57 mixed drinks (4 bottles of spirits at 17 shots each, minus a margin, so buy 4) plus about 7 litres of mixers. And roughly 90 lb of ice — the ice calculator explains why that number always shocks people.
100 guests, 5 hours (wedding-scale)
This size deserves its own treatment — toast rounds and dinner service change the math. See wedding drink quantities.
Adjustments that actually matter
- Crowd style. A university reunion is not a work function with the CEO present. Scale ±20–25% — the calculator has a toggle for exactly this.
- Heat. Hot outdoor parties push beer and spritz consumption up ~20% and wine down. People chase refreshment, not alcohol.
- Daytime vs evening. Daytime events run about 25% lighter across the board.
- What doesn't matter: obsessing over the split. If you run out of wine but have beer, people drink beer. Running out of everything is the only real failure.
Always round up, and here's why it's free
Unopened bottles and cans keep for months to years, and many bottle shops offer sale-or-return on unopened stock for events — ask; it turns overbuying into a free insurance policy. The asymmetry does the deciding for you: leftovers cost nothing, a dry bar at 9pm costs the party.
Run your own numbers in the party drink calculator — it applies everything above and gives you a copyable shopping list.