90 gallon soil sub

[loachy dance]

Im trying to set up a soil substrate with gravel top and im wondering how much soil i need for a 90 gallon in terms of pounds. Ive got no clue on this.
Im thinking an inch and a half deep?

 

[Mattfinn]

Volume formula=LxBxH
Just set the height at two inches.
Then convert the inches cubed or cm cubed to mass.

 

[loachy dance]

Yeah, but does anyone add slope into this? Also i guessit isnt supposed to touch the edges, but i can just subtract that..