Numbers aren't the only thing to keep in mind when you're ordering food for a party. Not all party guests are created equal. If you're inviting a bunch of 20-year-old men, you'll need much more food than a quiet cocktail party for older friends. If children account for some of the head count, you'll need less food and drinks overall.
If you're throwing a cocktail party where the only food you're serving is appetizers, take the time of the party into account. At a cocktail party held during dinner time, plan on each guest eating between 15 and 20 appetizers and order around 300 appetizers. If the party is being held mid-afternoon or you're going to be serving a sit-down dinner as well, plan on guests eating five or six pieces each, so order between 75 and 100 appetizers.
If you're planning a sit-down dinner, start by estimating how much meat you'll need. According to the Old Farmer's Almanac, 12½ pounds of turkey or chicken serves 25, so around 10 pounds should serve your crowd. If you're planning a barbecue, buy seven or eight pounds of hamburger or around five pounds of hot dogs to feed everyone. Make a salad out of four heads of lettuce and buy seven to eight pounds of potatoes for side dishes.
By the time dessert is served, guests might be slowing down and getting full. For a crowd of 15 to 20 people, two cakes or three pies is sufficient. If you're serving individual cookies or pastries, plan on each guest eating two or three and order around 40 to 50. At a cocktail party where guests won't be eating a full meal, they might have a larger appetite for dessert so buy double the number of desserts.
Unless you're inviting only non-drinkers or you're throwing a lunch time party, guests will appreciate being offered wine or beer. For 15 to 20 adults, Epicurious recommends buying seven bottles of white wine, two bottles of red wine and 50 cans or bottles of beer. If you want to serve liquor as well, buy two bottles each of vodka, gin and rum. Buy four to six two-liter bottles of soda along with two or three bottles of orange or cranberry juice. Finish the bar with around 15 pounds of ice.