How many gallons in standard BBQ propane tank and can it be overfilled?.?

A standard BBQ cylinder has 20lbs of propane in it (give or take a little for manufacturing differences). There is 4.2 Lbs of liquid propane in a gallon so 20 Lbs is 4.77 gallons....and Yes! A cylinder can definitely be overfilled! It shouldn't be! but it can happen. Most 20 lb cylinders today have an automatic stop fill device that shuts off the liquid flow into the tank at around 80%. But they can and do fail. That is why cylinders are still filled on scales and weighed to ensure that you are not receiving an overfilled tank.