First, figure out how many drinks you'll need for the duration of the party. As a general rule, assume guests will have two drinks during the first hour of the party, and one drink every hour after that.
Number of guests x Estimated number of drinks per guest = Total number of drinks
Next, figure out how you want to divide those drinks among wine, beer and liquor, depending on the crowd you're serving. Or to make it simple, just allocate a third of the total number of drinks to each type.
Wine: A 750-ml bottle of wine contains about 5 servings, so divide the number of wine drinks by 5 to come up with the number of bottles you'll need. (If you don't need to take into account other types of alcohol, an even easier way to estimate wine needs for a sit-down dinner is to assume half a bottle of wine per person.)
For Champagne or sparkling wine, a bottle will fill about 6 flutes.
Beer: For large parties, a keg often makes the most sense. Unfortunately, keg sizes are not standardized, but in general U.S. half-barrel kegs are 15.5 gallons and quarter-barrels contain 7.75 gallons which, if you are using 10-ounce cups, equals about 200 servings and 100 servings of beer, respectively. Take a look at these charts on Wikipedia for more U.S. keg size specifications and European keg size specifications.
Liquor: Mixed drinks have a 1.5-ounce (45 ml) serving of liquor per drink, so a 750-ml bottle will make about 16 drinks. To figure out how many bottles you need, just divide the number of liquor drinks needed by 16.
To estimate the amount of mixers needed, figure about 1 quart (1 liter) of tonic water, soda water, or juice for every 3 guests.
And don't forget to round up and, if possible, buy a little more than you think you'll need. Check with the vendor you buy from; some will give refunds for unopened bottles.
Do you have any tips for calculating how much alcohol to buy for parties?
(Image: Nora Maynard)