Propane weighs around 4.2 lb per each gallon of Propane so if you have a 20 lb propane cylinder and it's been filled up fully then the 20 lb propane cylinder should contain 4.7 gallons of propane in the 20 lb propane cylinder.
Since it takes 4.2 lbs of propane to equal a gallon of propane you would have less than a gallon of propane in 1 lb of propane.