Well having done this once to work out substrate depth on my Vision 180. (rounding figures as we go).
Vision 180 area is 92cm x 45cm which gives an area of 0.92 x 0.45 = 0.4m2.
Your depth is 2.5in inches which is 6.3cm.
This gives a volume of 0.4 x 0.063 = 0.025m3.
According to Eco-complete web site its density is 54lb/cu ft, which is 864kg/m3.
Thus weight needed - is 0.025 x 864 = 22Kg.
Thus you will need at least 3 bags (27Kg), possibly 4.