gravel for 90 gallon tank

[HeatherW]

how much gravel do I really need for a 90 gallon tank. I know the rule of thumb is something like a pound per gallon, but a 75 and a 90 have the same bottom footprint. so do I need 40lbs, 60 lbs, or more like 80 lbs??

I am looking at mixing a couple different colors to make my own mix and can't figure out how much I need to buy.

 

[TWEDM1]

Get 20 to 25 bags attest 100 pounds. Better to have more then not enough

 

[TWEDM1]

Sand is good too.. Black sand is sexy in a tank

 

[HeatherW]

Get 20 to 25 bags attest 100 pounds. Better to have more then not enough

so you really think I need that much? is the rule so many inches on the bottom of the tank? I can't remember what the exact formula is