It would take approximately 10 pounds of dry pasta to feed 40 people as a main course. The exact amount it would take depends on the type of pasta being served, the size of the portions and the sizes and ages of those eating it.
In general, it takes 4 ounces of dried pasta to make one serving. However, the volume of cooked pasta depends on the type and shape of the pasta being cooked. A pound of dry macaroni or similarly shaped pasta will produce 9 cups when cooked, while a pound of dry spaghetti or linguine produces 7 cups of cooked noodles. Egg noodles expand less when cooked than do other types of pasta, and fresh pasta does not expand at all.
Two ounces of dried pasta is sufficient per serving if served as a side dish or a first course, so 5 pounds of dried pasta would be enough to make 40 servings. Portion sizes should be dependent on the those eating the pasta. Teens tend to eat between 30 and 40 percent more than the average diner, and senior citizens tend to eat less. Cooking the pasta takes 4 to 6 quarts of boiling water per pound, and a tablespoon of salt per pound of pasta cooked is often added to the cooking water.