We fill your gas grill tank to the full 20 lbs of propane for which your tank was designed. You do NOT get a full 20 lbs when you take it to an exchange center. In a propane tank exchange, they only put 15 lbs. in your tank (5 lbs. LESS than the capacity of the tank!) Since this is not widely known, it often seems better to exchange...but it is always a better use of your money (and time) to have your tank filled at Rent-A-Wreck Erie