They recommend 1#/5 cu. ft. Measure the box in inches L,W,H and divide each by 12. Then multiply all 3 and you have cu. footage.
ex.
16"/12=1.33'
12"/12=1.00'
14"/12=1.17'
1.33' X 1.00' X 1.17' = 1.55 cu. ft.
So in this case I would use 1.55/5 or .31 pounds