From AmeriGas Website
Review the propane tank size guide below for more information on propane tank sizes. It is important to note that a propane tank is only able to be filled to 80%. The tanks details will highlight how many gallons you get when full.
20 lb Propane Tank
Dimensions: 1' 6" in height and 1' in diameter.
Uses: Propane grills, generators, patio heaters, and other outdoor equipment. To purchase or exchange a 20 lb tank, find a location near you.
20 pound propane tanks are often referred to as grill cylinders and hold 4.6 gallons of propane when full.
From another website: 1 gallon of propane weighs 4.11 lb.
Therefore; 4.11*4.6 = 18.9 lbs of propane plus the empty tank weight.