20000 feet

What Is The Atmospheric Pressure At 18000 Feet? Pressure on average will decrease by nearly half for each 18,000 feet of altitude. The ISA surface pressure is 29.92 in. Hg (1,013.25 mb) or 14.7 pounds per square inch, and at 18,000 feet it will decrease by half to about 14.94 in. Hg (500 mb) or