Atmospheric density at sea level is 1.225 kg/m3. Density of air decreases with height/altitude. According to International Standard Atmosphere (ISA), for an altitude of 40,000 feet, the density of air is 0.003996 kg/m3. So the aircraft operating at 40,000 feet msl will encounter an atmospheric density of 0.003996 kg/m3 (0.000249 lb/cu.ft.)
