I was looking for the same the other day. As the ADA Aquasoil is in bags marked in liters. Take the dimensions of the tank by the thickness you want.
Take that dimension and multiply is by the number below and that is what you need in liters. Then take that number and divide it by say 9, for the 9 liter bag and that's how much you need.
(for my 125g tank) 72"x18"x3" = 3888 cubic inches
1 cubic inch = 0.01638 Liters
3888 x 0.1638 = 63.685 liters
63.685/9 = 7.07 bags of 9L Aquasoil
FYI I'd be looking at over $200 for this. Then if you get power sand too, that's even more $$$$!!
I wanted aquasoil, but not sure what I am going to do now...