Determine the cubic feet of your concrete slab by multiplying the length x width x depth in feet. (If your depth is less than one foot, divide the number of inches by 12 to determine the depth in feet. E.g., 4 inches ÷ 12 is 0.33 ft.) Once you've figured out the cubic footage of your slab, divide it by 0.60, this is the number of cubic feet one 80 lb bag of concrete mix yields.