if you think about it it will make sense.....i really suck at explaining things but trust me this is how its done.....immagine a cubic foot of water weighs 40 pounds and 1 cubic foot is 1/3 scale compared to a cubic yard, you would multiply 3x3x3 to get 27 (27 cubic feet in a cubic yard) then take 27 and multiply it by the weight of a cubic foot of water being 40lbs....so 27x40=1080....i hope this clears things up, like i said im really bad at explaining things