How do you calculate volume of pasta from its weight after cooking down? I am hoping to make two 3 quart casserole dishes worth of macaroni and cheese. The recipe I have calls for using two pounds of pasta, but doesn't indicate the volume of food that it will yield after cooking down, or after combining with sauce.

Assuming I will use either small shells  or fusilli (haven't bought the pasta yet because I don't know how much I will need), how do i translate poundage of pasta to volume?
Because of the different shapes, different pastas will occupy different volumes of space both before and after cooking.

There are two answers that readily occur to me, one simple one more complex.

The simple solution, of course, is just to make too much.  Cook your pasta, put in the casseroles with sauce or adding the sauce after, depending on your recipe, and if you have too much (you should, you planned for it) let it cool, add mayo, mustard, onion, some pickle relish and make pasta salad.  Or throw in soup.  Or ....

Option two is to measure your pasta by volume when dry, assuming that it approximately doubles in size.  So you'll need to fill one of your casseroles with dry pasta, and that should give you enough to make two casseroles worth, once you have cooked the pasta.