Individual meats, fish, and poultry:
5-6 ounces protein per person
Take bones into account and add more weight if necessary. Don't worry about the loss during cooking unless the meat has a lot of fat and it's going to really shrink. Then estimate a little higher.
For multiple-meat meals and buffets:
4-6 ounces protein per person
I still estimate a similar quantity per person even when I'm serving more than one protein, because people will taste some of everything.
For a sit-down dinner, a pound of pasta will serve 4-6.
For a buffet, a pound will serve 8-12. I double the normal amount of servings for a buffet, because people won't eat as much pasta when they're filling their plates with other foods as well.
With a premixed salad, estimate one handful person.
One head of lettuce will feed about 5 people, taking into account different sizes--four medium heads will serve 15-20. I can't stress enough that you should think about how much you serve when you buy for your own family. If you buy a head of broccoli, how many servings do you usually get out of it? Some people cook the stems, other discard them.
For cocktail parties--finger foods, hors d'oeuvres, and appetizers:
Professional caterers estimate 10-12 items total per person.
Make 3-4 items per person of the more complicated hors d'oeuvres. For the easier things, make many more. When I throw parties, I take puff pastry, put curry paste in the middle, roll it up into pinwheels, and bake. I make a lot, because they're easy, unlike frying all your own tortilla chips. And remember, the estimate of 10-12 items might not work if people know you're a serious cook, because no one will eat before they come to your party and they'll all be starving.
Calculate one full portion of dessert per person.
For multiple desserts, people will taste smaller portions of more items. A cake that serves 10 will yield 15 portions if you're also having a tart.
Cooking For A Crowd:
If a recipe serves four, and you want to prepare it for 40, you simply multiply the quantities by 10, right?
Wrong, a culinary editor in the Food Network Kitchens who also taught Food Production and Management at NYU, where she showed culinary professionals how to create meals for large groups states "It's not as easy as just multiplying". You have to make other adjustments. You can't use 10 times the oil and spices--some things have to be added to taste, or eyeballed. If the original recipe called for 1/4 cup of oil, you have to take it on trust that not you're going to use, say, 3 1/2 cups of oil.
It gets even trickier with leavenings--they're all a pain, let's say a cake makes 10 to 12 servings. For 40 people, double the recipe and make two batches of the double recipe. That way you'll have a much higher success rate. You can do them simultaneously if you have enough pans and if the leavening agent is stable--something other than beaten egg whites alone, for example.
Doubling and tripling recipes is something of an art, so it's best to start smaller and work your way up to really large events. If you've never had more than four people over for dinner, don't plan to cater your own cocktail party for 50.
But if you've entertained on a larger scale and you want to expand with sit-down dinners for 12, buffets for 20, and cocktail parties with hors d'oeuvres for 35, there are some basic rules for figuring out how many pounds of tenderloin, chicken wings, and T-bones to order.
Experience is the best barometer, it’s important to remember that not everyone is going to eat everything. And people eat more starches and meats than vegetables at parties. And if there's anything fried, it's going first.
And don't forget the buffet rule: people feel the pressure of others behind them, so they move along swiftly and won't take a whole big chunk of food on the first trip.
If you're like most home cooks, you'll probably overestimate your portions no matter how careful you are. And that's okay--it's bound to happen with the sort of generous personality that likes to throw big parties. It's better to have a groaning board and leftovers than to have a sparse spread at a party.