Can someone help me with this calculation?

[trevorc13]

BIAB - 14.7 lbs of grain, 18.37 quarts (4.59 gallons) of water. How big of a brew pot do I need for this batch sparge BIAB? Beersmith says a 5.5 gallon mash tun! I doubt that 14+ lbs of grain adds only a gallon of space. I have a 7.5 gallon aluminum pot and I hope this will work. If not ill have to go old school with my cooler. Thanks for the help.

 

[VTBrewer]

I use a 5 gallon igloo cooler, and I can mash in at 1qt:1lb up to 14 lbs (and 3.5 gallons). I did two batches Sunday that were 12 pound/3gallons and both had efficiencies of 75-78%