To calculate how much you need, multiply the lenght of the tank by the witdth by the desired height of the substrate and devide by 1000
For example for an 80cm long and 40cm wide tank to achieve 2inch(5cm) substrate you'd need:
80x40x5= 16000/1000=16kg
And for 50 cm width
80x50x5=20kg
If you'd like an inch and a half, then depending on the relevant width where 1.5 inch is 3.81cm
80x40x3.81=12.2kg
80x50x3.81=15.2kg