Coffee for a crowd goes wrong in two ways: a pot that runs dry halfway through the evening, or three litres of cold, stewed leftovers. Both come from guessing. The sums are short, and once you know them you can plan any gathering, from six friends at brunch to forty people at a family party.
This piece gives you the two numbers behind every batch, a calculator that turns a guest list into grams and litres, and the habits that keep a big batch tasting like coffee for the whole event.
The first is the ratio. The Specialty Coffee Association's Golden Cup standard for filter coffee is about 55 grams of ground coffee per litre of water, with a tolerance of roughly 10% either way (so 50 to 60 g/L). That is around 1 part coffee to 18 parts water by weight. Go to the lower end for a milder pot that suits people who drink several cups, and to the upper end if the coffee will be diluted with milk.
The second is easy to forget: ground coffee soaks up water. A rough rule is that the grounds keep about 2 ml of water per gram, so a machine does not give you back everything you pour in. If you want 3 litres in the cups, you need to put in a little over 3.3 litres. At large volumes that is the difference between a full jug and a short one.
Plan by cups drunk, not by guests invited. Not everyone drinks coffee, and the ones who do drink more than one.
Set the guest list, how many of them will want coffee, and how long the event runs. The cups-per-person presets are planning rules of thumb, not measured data: people drink more at a long afternoon than a quick coffee after lunch. Change them to match your friends.
A single giant batch tastes worse than two smaller ones. Filter coffee starts to lose its aroma as soon as it is brewed, and a pot sitting on a hot plate keeps cooking. Common advice is to treat about half an hour on a hot plate as the limit. A pre-warmed insulated jug or pump pot keeps coffee hot without more heat, and stretches that to an hour or two before it turns flat.
Set out milk (warm if you can), sugar and a pitcher of hot water. A second, smaller pot of decaf is worth it when the event runs into the evening: it keeps the late guests from lying awake. Mention it on a card, so nobody has to ask.
On caffeine: European Food Safety Authority guidance puts the amount most healthy adults can have across a day at about 400 mg, and about 200 mg in a single go, with a lower limit of 200 mg a day in pregnancy. That is around four 200 ml cups of filter coffee over a day, so a party is not usually a problem, but it is worth having water and an alcohol-free alternative on the table too.
If you can plan a day ahead, cold brew takes the clock out of it. Steep coarse coffee in cold water in the fridge overnight, filter it, and you have a concentrate that keeps for a few days and can be poured over ice. Concentrate ratios of roughly 1 part coffee to 5 to 8 parts water are common; you then dilute it with water or milk to taste. Because it needs no heating, it is the easiest way to serve a lot of people, and it is good with a splash of tonic or soda for something that feels like a drink.
Whatever you brew, give people a way to say "just a small one". Half the waste at any event is the full cup poured and not finished.