Calculations.

To calculate the Mass of plates (from 4.5mm thick to 300mm thick)
Length (in metres) x width (in metres) x thickness x 7.85
e.g. 2500x1200x12x7.85 will give you 282.6 kgs as the weight for one 12mm plate

To calculate the Mass of sheets from 0.4mm to 4mm -
Length (in metres) x width (in metres) x thickness x 8.039
e.g. 2450x1225x3x8.039 will give you 72.38 kgs for one 3mm sheet