None, since there can be no such conversion.
A pound is a measure of mass. A cubic foot is a measure of volume. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid. If you are not convinced, consider a cubic foot of water. How many pounds? Next consider a cubic foot of lead. How many pounds?
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There question has no sensible answer. A pound is a measure of mass. A cubic foot is a measure of volume. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid. Consider a cubic foot of air. How many pounds? Next consider the same volume of lead. How many pounds?
1 ft = 12 inches so 1 cuft = 12*12*12 =1728 cu inches
Then, 20 cu ft = 20*1728 = 34560 cu inches