Party planning can be a headache, especially when it comes to deciding what to buy in advance. You don't want to run out of drinks before the party ends. Equally, however, you don't want to waste money and space by buying too much to drink. Estimating what you'll need is easier once you've determined a few basic facts about your guests, their drinking preferences and the type of bar you intend to host.
Determine how many guests you expect at your party. You may want to separate them into drinkers and non-drinkers, if you know that information; for the purposes of determining how much alcohol to buy, the number of drinkers is the most important piece of data.
Estimate how long you intend your party to be. It isn't always possible to be precise -- a good party often lasts as long as it lasts -- but give your best estimate.
Assume that the typical party guest will drink two drinks per hour for the first two hours of the party and one drink per hour for each hour after that. During a four-hour party, for instance, an average guest would consume six drinks. Multiply this number by the total number of guests to arrive at the number of drinks you'll need.
Decide on the proportion of different drinks you'll be serving. It helps to know your guests and their preferences. Different caterers suggest different guidelines, but they generally suggest a slight preponderance of beer, such as a 4:3:3 ratio of beer, wine and mixed drinks.
Divide your total number of drinks into these three categories. For instance, if you have 50 guests coming for a four-hour party, you will want 300 drinks in total: 120 beers, 90 glasses of wine and 90 mixed drinks.
Translate the required number of drinks into a number of bottles. A typical bottle of wine should provide about five glasses, while a bottle of liquor contains just over 20 servings. However, you'll probably need to provide multiple different types of liquor. You may need to round up to reflect this. The 50-person party example would therefore need 120 beers, 18 bottles of wine and about five bottles of liquor. Again, the actual number of liquor bottles might need to be increased so you don't run out of one type of liquor. Epicurious recommends providing vodka, rum, tequila, gin and three kinds of whiskey.
Include mixers and non-alcoholic drinks. Soft drinks, fruit juice, tonic water and other mixers are needed for a variety of mixed drinks. Additionally, of course, you'll want to have enough of these for your non-drinking guests to enjoy.