Our awesome alcohol calculator will determine your needs! It determines how many bottles of wine, beer, and liquor you will need based on your specific guests.Essentially, It's three drinks per person per hour during cocktails, then one drink per person per hour for the rest of the night. The easiest way to calculate the cost is that usually the budget for an open bar is between 10 to 20 percent of the overall budget for the wedding. We had a 120 person wedding and on the Zola RSVP we asked people their preferred drink then based the percentages directionally off that. Calculate how much Alcohol you need for your wedding, event, or party with the fun and easy With A Twist Alcohol Calculator. The Wedding Alcohol Calculator calculates how much alcohol to buy for a wedding or party, and how much buying the alcohol will cost. Ready to buy booze for your wedding? We have a wedding alcohol calculator as well as detailed advice on how to figure out what you need. The average wedding keeps the bar open for up to 5 hours.