The way I understand it, those 20lb propane cylinders are made to contain 20lbs of propane. That's just the weight of the propane, not the tank itself. Is this correct?
I just noticed that the local dollar store has exchange tanks, and a sign says "Net contents 15lbs". That would mean they are only 3/4 filled. I know some companies do this, which is really unfair, but they know most people dont read the signs, and as long as it's posted, they can get away with it.
The propane company is "Amerigas".
After reading that, I took my tank to a company that will refill them with the actual capacity.