How much alcohol to buy per guest
The planning rule bartenders use is one drink per drinking guest per hour. The first hour usually runs a little higher as people arrive and get a drink in hand, then it settles. For a four-hour party that is about four drinks per drinking guest, and the share who actually drink is rarely everyone, so the drinkers slider keeps you from buying for a room where a third are driving, pregnant, or simply not drinking.
A standard serving is twelve ounces of beer, five ounces of wine, or a one and a half ounce shot of spirits, and each of those carries the same amount of alcohol. That equivalence is what lets the tool convert a target number of drinks into bottles, cans, and cases regardless of how your guests split across beer, wine, and cocktails.
Splitting beer, wine, and spirits
A balanced mix for a general crowd is roughly half beer, a third wine, and the rest spirits, but the event should bend it. Weddings and dinners skew toward wine, holiday parties and cocktail-forward events lean on spirits, and a summer cookout is mostly beer. Buying to the mix that fits your crowd means fewer leftover bottles of the thing nobody reached for.
Always buy a little long on the format that stores well. Unopened wine and spirits keep for the next occasion, so erring up there costs nothing, while running out mid-party is the failure you actually want to avoid.
Ice, mixers, and the non-drinkers
Ice is the most commonly forgotten item. Plan about one to one and a half pounds per guest, because it does double duty chilling bottles and cans and going into glasses, and it disappears faster than people expect on a warm day. If you are serving spirits, the mixers and garnishes need their own line, and the soft drinks for non-drinkers and kids are not optional: a third of many rooms is not drinking alcohol, and they still need something in hand.