Its heavily dependent on the type of rock used. You are really after volume. Weight has zero to do with it, but is used as an estimate. I dont know why sellers dont go by estimated volume? For instance, concrete is going to weight much much more than fiji rock. Porous and high volume is what you want. You may want to ask a seller how much space a certain weight takes up before you buy their rock. They usually charge you for the water weight too, so keep that in mind. I paid for 15lbs of fiji rock and got about the equivalent of 9lbs of dry rock.
So in my case, id need twice as much (30lbs ) for my 20g.