Volume Required for grain?

I'm tying to do some calculations for how much liquid I can add to my 5 gal cooler MLT for a mash. What I don't know is how much volume a pound of grain takes up. In other words, if I have say 10lbs of grain in my MLT and fill my MLT up to the 5 gal mark, how much water does it take?
I know I can't get 5 gal of water in the MLT on top of the grain, but can I get 4 gal? 4.5gal? less, more?
Any ideas?
Thanks, I am trying to figure out my limits with my 5 gal system. I can't really boil more than my 5 gal MLT will produce and I don't have a problem with adding extract for a bigger beer but I am trying to figure out what my limitations are so I can plan recipes.
Craig

15 lbs of grain and ~3 gals of water is the max i've fit into my 5 gal igloo cooler, my efficiency is usually about 65-70%, so this example was overkill on the grain for the hefe i was making. Generally I go for about 12 lbs of grain and ~3.5 gals of water, plenty of folks hit the target gravity with only 10 lbs though so play around a bit to find your efficiency.