The tank's tare weight is stamped on its side. That's the weight of the empty tank. If it reads "20" and your bathroom scale reads "25" there's 5 lbs of propane left in the tank. As propane weighs circa 4 1/4 lbs per gallon, 5 lbs left in your tank means you have a tad over 1 gallon of propane left in the tank.