A cubic foot (ft3) was a unit of volume defined as a cube measuring one foot per side.
When "Professor" Montgomery Scott approached Doctor Nichols regarding his products, he asked, "How thick would a piece of your plexiglass need to be, at sixty feet by ten feet to withstand the pressure of 18,000 cubic feet of water?" Nichols quickly replied that it would need to be six inches thick. (Star Trek IV: The Voyage Home)