If you find yourself in the position of cooking and shopping regularly for a large group of people, the pamphlet "Family Food Buying" (USDA Home Economics Research Report #37) provides a concise and reliable guide to quantities needed for purchasing. Also, the recipes on this site, developed over many years of cooking for groups of 25 and more, will give you ideas for the occasional gathering, class, or reunion meal.
Equipping a permanent group kitchen
If you are purchasing equipment for a group kitchen, shop at restaurant supply houses rather than hardware stores or department stores. The equipment there is designed to be durable and useful without being over-priced, and is sometimes available used. The following are "must haves" you want to add to your regular equipment to make cooking so much food a little simpler:
Heavy stainless or cast aluminum stockpots--minimum, one 12 quart for 15-20 people, or 32 quart for 70. After you find out what kinds of food you cook regularly, get others from 8 quarts on up as affordable.
Covered metal oval or rectangular roasters-- largest that will fit covered into your oven with the bottom rack in, minimum 1 cup capacity per person. This will be used both in the oven and on top of the stove for cooking grains, vegetables, noodles, etc., and even soups and the occasional roast or turkey.
A large steel or aluminum colander--the type with a ring base.
Several solid and slotted long handled spoons. For safety, use single piece spoons, not those with a separate handle.
Ladles--several 1 cup, a ½ cup, for cooking and serving. A couple of small ones for sauces and dressings.
2 carbon steel French chefs knives, one about 8" and one at least 10-12", for each cook, and a sharpening steel--if you don't know how to care for and use them, find someone who does to teach you how. If more than one person is doing prep work, each person needs a good knife.
2 or three 5-6 quart Dutch ovens with covers--depending on the size of the group, these will be mere saucepans or usefully sized cooking pots. Cast iron or enamel pots with metal handles have the advantage of being oven proof, and can also be used as casseroles. Glass lids allow you to save heat by checking the contents without lifting lids, and can be replaced at any Corning/Revere factory outlet. One Dutch oven holds about 12 large bowls of soup or 16-20 servings of rice or grain.
1 15" wire whisk, others as affordable
metal egg beater
griddle-- a two burner rectangular griddle
frying pans-- at least one of the largest size you can use on your stove, and the heavier the better
1 kitchen fork strong enough to turn 12 lb. roast
1 large pancake turner
kitchen shears-- usually red or brown handled, the kind that come apart for cleaning
metal mesh strainers, 3 inch for tea and small jobs, 9 inch for hot things and pots full
Wire cookie racks that will fit to convert the roaster to a steamer
In addition, depending on the type of cooking you are doing you may need:
Loaf pans-- up to twice as many as your oven will hold. The 4 ½"x 8 ½" size is preferable to the standard 5"x 9" for most baking
Cookie sheets-- at least three, the flat type without sides is easier to store and doesn't warp. Get the largest size that will allow 2 inches on each side between the sheet and the oven wall.
Mouli graters-- a small drum-type one for garlic and garnishes and a standing model (about for carrots, cabbage, cheese, potatoes, etc.) Try to avoid the vacuum base cone-type graters: I've never used one that worked as well as the Mouli, and the vacuum base often comes unstuck. There is an electric model grater called "the presto salad shooter professional model" that is handy, but don't get the regular one, as it has a too-small feeding tube.
Pie plates--if your oven will hold two on one rack, the 10" glass ones by Corning are most practical. They make 10-12 slices each and can be baked 3 at a time (2 above and 1 below) in most ovens.
Casserole/cake/baking pans--the stainless steel 13"x20" size, which will cut into 24-40 pieces, is most practical (again, if it will fit in your oven). 2 or more depending on the size of the group, for cakes, custards, cornbread, quiches, etc. Be sure you do not buy the stainless steel steam table trays which are about the same size; they are almost useless for baking.
Muffin tins--3 12-cake pans is enough for 20 people, more as necessary
Griddle--if your stove doesn't have a built-in griddle, a rectangular 2-burner griddle for grilling and frying
tube pans--two or more for cakes and molds
Mixing and storage containers--the most practical large mixing or storage container I've found is a clean 30 lb. mayonnaise tub or white plastic 5 gallon food storage drum . These are usually available from restaurant or quick food places, have tight-fitting lids, and do not dent or rust. Properly sealed they are also almost impervious to bugs, water, rodents, and dust. They make good bread-rising containers, too, being large enough to rise a 6-8 loaf recipe and airtight enough to keep it moist. Also, they stack easily and can be sat on. Food safe bussing bins (grey, made by Rubbermaid) are huge, safe for mixing saalds, and lids can be purchased separately.
Good smaller containers include 5 lb. cottage cheese containers (especially useful because they hold precisely 10 cups), gallon glass or plastic jars, honey cans with snap lids, coffee cans with snap lids, etc. Only in rare cases should your need to buy a special storage container.
Other impromptu mixing containers include clean dish pans, vegetable bins, or crisper drawers. DO NOT USE any old plastic container! Plastic containers that are not normally used for food-related purposes may leach undesirable (poisonous) chemicals into the food.
Green or brown glass 8 oz. wide mouth beverage bottles with corks added make good large size light-resisitant spice/herb containers.