Brewing coffee for a crowd is a scaling problem with one correct constant and two traps. The constant is the specialty-coffee "golden ratio": 60 g of ground coffee per litre of water. The traps are cup size (event cups are 180 ml, not your 350 ml mug) and timing (urns brew slowly, and a queue forms the moment the first one runs dry).
How many cups people drink
| Time of day | Cups per guest |
|---|---|
| Morning (meetings, brunches, funerals) | 1.25 |
| Afternoon | 1.0 |
| Evening (dinner service) | 0.6 |
Cold weather adds ~15%; an outdoor summer event halves demand and you should offer iced alternatives instead.
The reference table
Morning demand, 180 ml cups, 60 g/L:
| Guests | Cups | Water | Ground coffee | Urn size |
|---|---|---|---|---|
| 20 | 25 | 4.5 L | 270 g | 5–6 L |
| 50 | 63 | 11.3 L | 680 g | 12–14 L |
| 100 | 125 | 22.5 L | 1.35 kg | 2 × 14 L |
| 200 | 250 | 45 L | 2.7 kg | rent a tower, honestly |
Milk, sugar and the rest
- Milk: ~50% of drinkers take ~30 ml. For 60 cups: 1 litre. Add a small oat milk — at most events it's now the first carton to empty.
- Sugar: ~1 in 3 cups takes one packet.
- Decaf: 10% of the total, brewed and labelled. Unlabelled decaf is a betrayal people remember.
- Tea: 1 bag per 5 guests alongside coffee — closer to 1 per 3 for UK/Australian crowds. The urn's hot water tap covers it.
- Cups: buy 15% over the cup count; people abandon and re-take cups constantly.
Urn technique
- Brew capacity should exceed demand by 20% — an urn can't be refilled mid-service without a 20-minute gap, which at a 50-person meeting means a visible queue and a ruined break.
- Start the urn 45–60 minutes before service. Large urns are slow; a 12 L urn takes about an hour to brew and reach holding temperature.
- Grind for the method: medium for drip urns and percolators; too fine and large urns turn bitter and silty by the bottom third.
- Held coffee degrades after ~90 minutes. For an all-day event, brew in two waves rather than one heroic morning batch.
The coffee calculator runs the ratio, urn sizing, milk and sugar for your exact headcount, in metric or US units.