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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Why every coffee guide gives you a different number
Search “how much coffee for 100 people” and you will get 5 gallons from one guide, 6.25 from another and 12.5 from a third — a 2.5× spread for the same party. Nobody is lying. Each guide silently assumes a different cup size (6 oz event cups vs 8 oz mugs) and a different number of cups per person (one polite cup vs two conference cups). Multiply hidden assumptions together and the totals fly apart.
This calculator refuses to hide them. It asks the only three questions that matter — how many of your guests actually drink coffee (event planners at Cvent put it near 60% for a morning group, less in the afternoon), how many cups each drinker will have, and how big your cups are — and then does the one honest formula: servings × ounces ÷ 128 = gallons.
The urn question, answered
Urns are sold by “cups”, but an urn cup is a 5–6 oz tasting cup, not your mug. A “100-cup” urn holds about 5 gallons; a “55-cup” about 3 gallons; a “30-cup” about 1.5 gallons. Plan in gallons, then translate. Brew in batches rather than one giant early batch where you can: coffee holds its flavour for roughly 1.5 hours in a hot urn, so for a long event the calculator’s batch count is a schedule, not just a capacity.
Decaf: the 1-in-5 rule
About 20% of coffee served at mixed events is decaf, which is why the calculator defaults to a 20% decaf split (weddings skew a little higher in the evening, morning conferences lower). Brew decaf in a smaller pot if you have one — a full urn of decaf is the classic leftover.
Grounds, in plain weight
At a regular 1:16 brew ratio (1 gram of coffee per 16 grams of water), one gallon of coffee needs about 0.52 lb of grounds. Caterers cross-check with a simpler rule — roughly 1 lb per 50 small cups — which lands in the same place. Buy whole pounds rounded up; leftover grounds keep, leftover brewed coffee does not.
Feeding the same crowd? Pair this with the Thanksgiving Turkey Calculator for the main event, and read the worked tables in our guide: How Much Coffee for 100 People?
Frequently asked questions
How much coffee do I need for 100 people?
With typical assumptions — 75% of guests drinking, 2 standard 8-oz cups each — 100 guests need about 9.4 gallons of coffee and roughly 4.9 lb of grounds. If everyone drinks only one small 6-oz cup, it drops to about 5 gallons; that hidden difference is why published guides range from 5 to 12.5 gallons.
How many gallons of coffee for 50 people?
About 4.7 gallons at the standard planning assumptions (75% drinkers, 2 × 8 oz cups), using about 2.4 lb of coffee grounds. For a light afternoon gathering with one cup each, plan closer to 2.5–3 gallons.
How much ground coffee per gallon?
At a regular 1:16 brew ratio, about 0.52 lb (237 g) of ground coffee per gallon of water. Stronger 1:15 brewing takes about 0.56 lb; milder 1:18 about 0.46 lb.
How much decaf should I make?
Plan on about 20% of the total as decaf for a mixed crowd — a little more for evening events and weddings, a little less for morning meetings. The calculator splits your total into regular and decaf automatically.
What size coffee urn do I need?
Work in gallons: a 30-cup urn holds ~1.5 gallons, a 55-cup urn ~3 gallons, and a 100-cup urn ~5 gallons (urn ‘cups’ are 5–6 oz). For anything over one urn, brew in batches — coffee stays fresh about 1.5 hours in a hot urn.