Everything converts through the standard drink
Beer, wine and spirits are sold in units that have nothing to do with each other, so the only way to move between them is through the alcohol itself. In the United States a standard drink is defined as 0.6 fluid ounces of pure ethanol, and three familiar servings contain approximately that: a 12 ounce beer at 5 percent, a 5 ounce glass of wine at 12 percent, and a 1.5 ounce measure of 80-proof spirits.
That equivalence is what makes the calculation possible. The page works out a total number of standard drinks for the whole event, splits that total by the mix you are serving, and converts each share back into the container it is sold in: one beer per can, five glasses per 750 mL bottle of wine, sixteen measures per 750 mL bottle of spirits. Every one of those conversions rounds up to a whole container, because you cannot buy nine tenths of a bottle.
The equivalence also breaks in one direction worth naming. A 7 percent IPA in a 16 ounce can is nearly two standard drinks, not one. A wine at 14.5 percent poured to the rim of a large glass can be two. If most of what you are buying is strong, the number of containers this page gives you supplies more alcohol than the total implies.
The rate, and why the hours taper
The three levels correspond to roughly 0.7, 1.15 and 1.8 standard drinks per drinking guest per effective hour. Those are shopping assumptions rather than measurements of anything, and the reason they are exposed in the results is so you can disagree with them knowingly.
The tapering matters more than the rate. People drink fastest in the first hour or two and then slow down substantially, so treating a six-hour party as three times a two-hour one badly overstates it. The model runs the first two hours at the full rate and everything after at sixty percent, which is why the totals grow sub-linearly with length. For a gathering where that is not true — an all-night event, a wedding with a long reception — add a quarter to the result.
| Hours | Effective hours used | Drinks per guest at the normal rate |
|---|---|---|
| 2 | 2.0 | 2.3 |
| 3 | 2.6 | 3.0 |
| 4 | 3.2 | 3.7 |
| 6 | 4.4 | 5.1 |
Guests who are not drinking
The count of people not drinking reduces the alcohol and does nothing to the soft drinks, which are calculated on the whole headcount. That is deliberate. Everyone drinks something all evening, drinkers included, and non-alcoholic volume is the line most often bought short.
The other half of it is quality rather than volume. A guest who is driving, pregnant, on medication or in recovery spends four hours with whatever you put out, and two bottles of own-brand cola is a different message than a decent tonic, a good non-alcoholic beer and something with citrus in it. It is a small amount of money and the most noticeable line on the list.
Mixers, ice, and the things that run out first
Ice is the item most reliably underestimated, because it does two jobs: chilling bottles and cans in a tub, and going into glasses. The figure here is roughly a pound and a half per person plus an allowance for the length of the event. If drinks are being built individually rather than poured from bottles, buy more, and if you are chilling a large amount of beer in a tub rather than a refrigerator, buy substantially more.
Mixers run at roughly three litres per bottle of spirits, which assumes ordinary highball proportions. Split it across what you are actually mixing with rather than buying three litres of one thing. Citrus, garnishes and something bitter are the other items that are cheap and conspicuous by their absence.
Rounding up on purpose
Every conversion here rounds up, and the result is intended to leave you with something over. That is not sloppiness; it reflects the asymmetry of the two errors. Unopened beer, wine and spirits keep indefinitely and simply become the start of the next occasion. Running out at ten in the evening ends the part of the evening people came for, and the shop run to fix it costs an hour and a driver. Buy the higher number.
Where returns are possible, and many retailers will take back unopened cases bought for an event, the asymmetry gets larger still — ask before you buy rather than after, since it turns the over-supply into something that costs nothing at all.
Questions people ask
How many bottles of wine do I need per person?
For a dinner where wine is the only drink, the common planning figure is around half a bottle per drinking guest for a first couple of hours and less thereafter, which is what the tapering in this calculator produces. A 750 mL bottle holds five 5 ounce pours, but very few people pour 5 ounces — a generous glass in a large modern wine glass is closer to 7, which turns a bottle into three and a half servings rather than five. If you are pouring generously, add a bottle for every three the calculator gives you. For a longer event with other drinks available, the wine share drops sharply and the mix setting handles that.
Does the strength of what I am buying matter?
Yes, and it is the largest source of error in the result. The conversions assume beer at about 5 percent in a 12 ounce serving, wine at about 12 percent, and spirits at 80 proof. A 16 ounce can of 7 percent IPA contains close to two standard drinks, so a case of them supplies nearly twice the alcohol the count implies. Wine has drifted upward too and 14 percent is common. If your list is mostly strong beer or high-alcohol wine, either reduce the container counts by around a third or accept that you have bought more alcohol than the total says.
How much ice do I actually need?
More than seems reasonable, because it is doing two separate jobs. Ice going into glasses is roughly half a pound per person per hour of serving. Ice chilling bottles and cans in a tub is a different quantity entirely and depends on how much you are chilling and how warm it is — a large tub can take twenty pounds on its own and will need topping up. The figure here covers a normal indoor gathering. For an outdoor event in summer, or one where drinks are built individually with ice in a shaker, double it. It is the cheapest item on the list and the most annoying to run out of.
Is the amount this suggests a safe amount to drink?
No, and it is not intended as one. This is a purchasing quantity for a group, calculated to be comfortably over rather than short, and it does not divide meaningfully into a per-person recommendation. Public health guidance on alcohol is set per person per day and is well below what the per-person arithmetic here implies for a party. The quantities are about what to have in the house; how much any individual drinks from it is a separate matter, and the things worth planning alongside the shopping are food, water, and how everyone is getting home.