Gravel Level

Last time I was at the lfs buying gravel, the employee told me to buy one bag of gravel per 10 gallons, which I did, and it worked out fine. Thing is, I can't remember how many pounds each bag contained.... So I don't know how much help this is to you :) But each bag was, if I recall, maybe about 10 inches by 6 or 7 inches, and maybe about an inch thick.
 
Depends on how much gravel you want. The weight per volume of water doesn't really work well - a 20 high has a smaller footprint than a 20 long yet you should put in the same amount of gravel? It also depends on the density of the substrate you are using. I only have 60lbs of gravel in one of my 150s because they're messy fish and I wanted it easy to clean, so its only about 3/4" deep. On the other hand, I used 50lbs of SMS in my other 150 and its 3-4" deep. Using standard gravel or sand, putting in 10lbs per square foot will usually get you a decent 1-1.5" gravel bed.
 
do a search for substrate calculator, i can't find the link but there are calculators available for figuring out how much substrate is needed fora desired thickness
 
I'd say about 50 lbs as well, as I like a little deeper gravel to get fake plants down into.
 
AquariaCentral.com