Steel Coil Weight Chart
which says, if I'm reading correctly, that a coil 12" wide and 48" in diameter (close to what I see on the road around here) weighs around 5000 pounds (2.5 tons).
No wonder you never see the trucks carrying more than two. Most of the time I see them with only one, but I have seen them with two, although it's been too long ago to remember dimensions.
I used my WEIGHT program to calculate the weight of a steel washer with the following dimensions...
ID = 20 in
OD = 48 in
thickness = 12 in
and it came back with ~18000 in^3 for a weight of 5089 lbs which is in very close agreement with the result obtained from the table. IOW, there isn't much difference between the rolled steel and solid steel which seems intuitively correct.