How much coffee for a crowd? A planner for cups, grams and brews

Coffee for a crowd goes wrong in two predictable ways. Either the pot runs dry halfway through dessert, or you pour half of it down the sink at midnight. Both come from guessing. Filter coffee is one of the few drinks where the arithmetic is simple, so you can plan it instead.

The planner below turns a guest list into litres, grams of ground coffee, the water to put in the machine, and how many brews that means. Move the sliders and the cups fill in.

The one ratio to remember

The Specialty Coffee Association's long-standing "Gold Cup" guideline for filter coffee is 55 grams of coffee per litre of water, which is about 1 part coffee to 18 parts water. It is not a law. Many people like it a little stronger or lighter, so the planner lets you move between 50 and 60 g per litre. For a feel of the numbers, 55 g per litre is roughly 100 g for a 1.8-litre brew.

Water temperature matters as well. The same guideline puts it at about 90 to 96 °C when it meets the grounds, and a total brew time of around four to eight minutes. Most home drip machines get close to this, but cheap ones can run cool, which gives a sour, thin cup. Hotter is not better either: boiling water will over-extract and taste harsh.

Why you put in more water than comes out

Dry coffee behaves like a sponge. The wet grounds hold on to about twice their own weight in water. So 100 g of coffee keeps back roughly 200 ml in the filter basket. Skip this and you end up a cup or two short for every full machine. The planner works it out for you: the "water in" box is bigger than the "litres brewed" box for exactly this reason.

Plan the cups, not the machine. Then check the machine can do it in the time you have.

How many cups per guest

A good starting rule for a dinner or a party is one to one and a half standard cups each for the people who drink coffee, and more for a long afternoon. Not everybody drinks it, so the planner starts at 80%. Move that up for an office or a family gathering with lots of regular coffee drinkers, and down for a late party where tea and soft drinks will be popular.

Cup size is the other trap. A "cup" on a coffee machine usually means a small measure of around 125 to 150 ml. A mug is closer to 250 ml. If your guests will hold mugs, choose the 200 ml setting, or tell the planner to leave some spare.

Keeping it hot without cooking it

Brewed coffee does not like being kept on a hot plate. The plate keeps heating the pot, and the coffee slowly cooks, turning flat and bitter. A well insulated flask or thermal carafe holds temperature for several hours without that problem. So for a crowd, brew in rounds, pour each round straight into a pre-warmed flask (rinse it with hot water first), and serve from the flasks.

That is why the planner shows flasks as well as brews. One litre of brewed coffee is around six to seven standard cups, so a 12-cup pot is a fine size for a small group, but for 30 people you want two or three flasks in rotation rather than one giant pot.

Caffeine and decaf

The European Food Safety Authority describes up to 400 mg of caffeine a day as unlikely to cause harm in healthy adults, and up to 200 mg in a single sitting. It puts a 200 ml cup of filter coffee at about 90 mg, though the real figure varies with the coffee and the brew. Some people feel the effects of even 100 mg, especially close to bedtime. So if your party runs into the evening, put a decent share of decaf on the table. Label the flasks clearly. Pregnant guests and anyone watching their intake will thank you.

The planner starts with 25% decaf. A slightly bigger share is not wasted: many people will take it after dinner anyway.

Variations: iced, milky and alcohol-free

For a hot day, brew the same amount but double the strength and pour it over ice, so the melting ice brings it back to normal. A tray of ice cubes made from coffee is even better, since they do not dilute the drink. For a milky crowd, plan on milk for about half the cups, around 30 ml each, plus a plant-based option. Coffee is also a natural alcohol-free finish to a meal, next to tea and a jug of cold water, so nobody is left holding a glass they do not want.

As ever, if anyone has had a few drinks, coffee does not sober them up. It does not make driving safe. Offer a bed, a lift or a taxi.

What to look for in a machine

Whatever you own, four things decide whether it copes with a crowd. First, tank size: you want a litre or more per brew so you are not refilling every few minutes. Second, a brew temperature in the range above; look for a maker that says so. Third, a thermal carafe or the ability to pour straight into flasks, rather than a glass pot on a hot plate. Fourth, a wide shower head or spray that wets all the grounds evenly, which matters more as batches get bigger.

If your machine is small, run it twice with the same measure rather than overfilling the basket. Fresh grounds each round beat reusing them, every time.