You took 1200 and divided by 144 which is inches in a square foot not a cubic foot.
1 foot = 12 inches
Volume L x W x H
In feet 1 x 1 x 1 = 1 cubic foot
In inches 12 x 12 x 12 = 1728 cubic inches (144 x 12)
1 cubic foot = 1728 cubic inches
1200/1728= 0.6944 cubic feet

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