There is a more accurate, but still not dependable way of determining this. It has more than just the one variable. It has size of fish, surface area of the tank, tank volume, and a chart seperating fish into groups according to length to aid in determining this. I forget how to do it, but teemed with information from this site about specific fish and the minimum tank sizes for them, this is far more accurate. You can google it, but I haven't had any luck. I may have found this in a book, it's been a while and I never really needed to use it.