A friend just asked me this question and I directed her to this Martha Party Calculator. Sarah used this formula for her wedding and we have used it for events we’ve catered and it has not let us down. Running out of alcohol at a party is never a good thing.
Party Calculator
Follow this guide to determine how much to buy. The primary rule: Buy plenty. You can use it later or return unopened bottles (check store policy before making your purchase).
Item and Quantity
Wine: One bottle per two people per hour
Beer: Two per person per hour
Spirits: One bottle per five people per hour
Mixers: Three bottles for each bottle of alcohol
Ice: At least one pound per person per hour
Martha Stewart Living, November 2009
xo,
Lydia
Lydia