But there are ways of arriving at a general estimate.
Caterers figure amounts based on a four-ounce pour. (Ever wonder how much a serving is for calorie-counting purposes? Four ounces.) Modern glasses, however, tend to be large. A four-ounce pour in a 16-ounce glass looks stingy. So figure a five- or six-ounce pour unless you are using small glasses. That means a 750-mil. bottle of wine will give you four or possibly five servings. I generally estimate a half bottle per person, knowing that some guests will drink more, some none at all. It’s always better to have too much than not enough.
1 750-mil. bottle = four or five servings
1 case of wine (12 bottles) = 48 – 60 glasses
1 bottle of champagne = six flutes (four ounces, which looks right in a flute)
1 case of champagne (12 bottles) = 72 flutes
1 liter = 33.8 ounces, or 22 1½-ounce drinks (40 drinks per 1.75-liter jug)
1 gallon = roughly 20 servings (six-ounce servings)
If you have bottles of water (yes, I know it’s ecologically wrong, but it’s a great convenience for a host—and for guests too; as the host at least you can make sure those bottles are recycled), figure two bottles per person.