The quick answer: for 100 guests, plan on about 9.4 gallons of coffee and 4.9 lb of grounds — if 75% of your guests drink coffee and each has two standard 8-oz cups. Change those two assumptions and the honest answer swings from 5 gallons to 12.5, which is precisely why the internet argues about this. Our free Coffee for a Crowd Calculator runs your own assumptions; this page shows the working.
The two hidden numbers behind every coffee argument
Every guide you have read picked a drinker share and a cups-per-drinker figure without telling you. Event planners who do this professionally (Cvent’s guidance is the usual reference) expect about 60% of a morning crowd to drink coffee, fewer in the afternoon. Layer cup size on top — a 6-oz event cup versus an 8-oz mug — and one guide’s “5 gallons for 100 people” (everyone, one small cup) and another’s “12.5 gallons” (everyone, two mugs) are both internally consistent. The formula itself never changes: guests × drinker share × cups each × cup ounces ÷ 128 = gallons. The arguments are all in the inputs.
The table: 25 to 200 guests
Using the standard planning assumptions (75% drinkers, 2 cups of 8 oz each, regular 1:16 brew strength):
| Guests | Coffee to brew | Grounds | Servings |
|---|---|---|---|
| 25 guests | 2.3 gallons | 1.2 lb | 35 |
| 50 guests | 4.7 gallons | 2.4 lb | 70 |
| 100 guests | 9.4 gallons | 4.9 lb | 140 |
| 150 guests | 14.1 gallons | 7.3 lb | 210 |
| 200 guests | 18.8 gallons | 9.8 lb | 280 |
Hosting something gentler — an afternoon party where half the guests drink one cup? Halve the table, roughly. A morning conference of committed coffee drinkers? The calculator’s Morning Conference preset (75%, 2 cups) is already close; nudge cups per drinker to 2.5 for an all-day office event and watch the urn plan grow.
Urns speak a different language — translate it
Urn sizes are quoted in “cups”, but an urn cup is 5–6 oz. So the big 100-cup urn holds about 5 gallons — enough for the standard 100-guest plan only if it is refilled once. A 55-cup urn holds about 3 gallons, a 30-cup about 1.5. Two practical rules follow: brew in batches for anything longer than ninety minutes (urn coffee tires after about 1.5 hours), and never brew the decaf in the big urn — a fifth of the total is decaf at a typical event, and it is the classic leftover.
Grounds, sugar, milk: the shopping list maths
Grounds scale with gallons: about 0.52 lb per gallon at a regular 1:16 ratio — which cross-checks neatly against the caterer’s rule of 1 lb per 50 small cups. For the supplies around the coffee, plan on cups at servings + 15% (spills and seconds are real), roughly 1.5 sugar packets and half a sweetener packet per drinker, and about half an ounce of milk or cream per serving — call it a quart and a half for the 100-guest standard plan. Buy grounds by the whole pound, rounded up: leftover grounds keep for the next event; leftover brewed coffee does not.
Run your own numbers: the Coffee for a Crowd Calculator — pick your event type, set guests and cup size, and get gallons, grounds, urn batches and the full supplies list. Feeding the same crowd? The Turkey Calculator handles the main course.
What you will need on the day
Brews 15 to 45 cups in about 28 minutes, with a ready light and a two-way dispenser — the urn behind most church halls, offices and wedding coffee stations.
A crowd-size canister of medium-roast ground coffee — roughly 3 lb, which covers the grounds figure in most of the plans this calculator produces.
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Frequently asked questions
How many gallons of coffee do I need for 100 people?
About 9.4 gallons under standard assumptions (75% of guests drink coffee, 2 × 8-oz cups each). The often-quoted 5 gallons assumes everyone drinks exactly one small 6-oz cup; 12.5 gallons assumes everyone drinks two full mugs. Your real answer sits where your crowd’s habits sit.
How much coffee for 50 people?
About 4.7 gallons and 2.4 lb of grounds at the standard assumptions — one medium (3-gallon) urn brewed twice, or a large urn with a top-up. For a light one-cup gathering, about 2.5–3 gallons is enough.
How much ground coffee per gallon?
About 0.52 lb (237 g) per gallon at a regular 1:16 brew ratio. Stronger coffee (1:15) takes ~0.56 lb per gallon; milder (1:18) about 0.46 lb.
How much decaf should I serve at an event?
About 20% of the total coffee as decaf suits most mixed crowds — a touch more for evening weddings, a touch less for morning meetings. Brew it separately in a smaller pot so the regular urn can be sized honestly.