Propane tank pressure ranges from about 100 to 200 pounds per square inch. The size of the tank does not influence the pressure that the propane is under.
The outside temperature and the temperature of the propane in the tank can cause pressure to fluctuate. Warmer propane means more pressure inside the tank. When refilling, it is important to remember that 20 percent of the tank should be left empty to allow for gas expansion should the tank become warmer. In very cold conditions, overfilling of the tank is a concern. If the tank is moved to an indoor area, the gas will expand and release propane rather than air via the pressure release valve. Thus, in a 20-pound tank designed to hold 20 pounds of propane, the actual propane capacity is a few pounds less.