I work for a Cost Consultant and recently came upon a rather interesting and indeed tricky situation.
I usually find that my estimate on the total duct weight is more conservative than the contractor's but in this instance it was the other way round.
As an example, for a 7 feet long Galvanized duct of size 74"x54", I came up with a weight of 247 Lbs. (assuming 0% wastage).
The contractor on the other hand came up with 95.57 Lbs. which he confirmed included some wastage.
Can someone comment on the above calculations as to which is more correct.