When trying to decide how much alcohol (or any type of beverage) for a party, a good host or hostess should plan ahead. Simply running to the store and buying drinks willy-nilly will not only waste money, but be a recipe for disaster.
Determine your total number of guests. If there is no RSVP, take the number of people that you invited and multiply it by .80 (80%). As a general rule, 20% of the people invited do not show up to a party. This gives you a good standard on which to base your calculations.
Now, consider the length of your party. In most cases, your guests will drink 2 drinks during the first hour and one per hour afterward. If you have a heavy-drinking crowd, you may want to up that number to a flat 2 per hour.
So, if 100 people are invited to a 4 hour party, you should need drinks for 80. 160 drinks for the first hour and 80 each hour afterward. That is a total of 400 drinks. If you purchase cans or bottles, the math is easy from there. If you are purchasing mixed drinks, consider a drink to be 8 ounces and do your math from there.
Bring a master list and a calculator with you to the store.
Consider buying in bulk for a large party - ask your favorite liquor or grocery store if there is a discount for buying larger quantities of items.
If providing beer for your party, it may be more cost effective to purchase a keg.
Don't forget to consider cups, napkins and ice into your budget.