Finding the number around 4.24 pounds per gallon on the web but this creates a mystery for me. I just bought a new 20 pound talk at Costco with a gauge on it. Took it to have it filled and the pump showed it took 5.7 gallons? That comes out to 24 pounds in a 20 pound tank? To top it off the gauge on the tank isn't quite at full either. The guy at the store says a used tank being refilled normally holds 4.6-4.7 gallons which puts you at the 20 pound number so are brand new tanks really larger than 20 but there is always some left in the tank when "empty"???