Propane tanks are filled to 80% capacity in order to allow for expansion of gas with changing temperatures. The gas will warm and expand and you want some space for the expansion in the tank.
If you did not have room, the gas would probably vent out the pressure release valve. This creates a very dangerous situation. Imaging if that gas vent were to ignite. It would be bad.
This chart is based on the propane tank starting with 80% of its’ capacity. There are many factors that could affect the actual amount of gas in the tank, but these are good approximations. When the gauge on the tank indicates a percentage left, here is what is left in gallons in the tank.
And for larger, home and commercial tank sizes, here is a reference table.
