I just cut and pasted this...

"There are 231 cubic inches in one US gallon. Since a cubic foot is 12 by 12 by 12 inches, a cubic foot has 1728 cubic inches. Thus one cubic foot = 1728 cubic inches = (1728/231) gallons = 7.48 gallons."

So 20 cubic feet times 7.48 gallons = 149.60 gallons in a 20 cubic foot bag.

However, I reserve the right to be totally wrong.