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.