As altitude increases, atmospheric pressure decreases because there is less air above pressing down. The relationship is modelled by the International Standard Atmosphere (ISA), which defines standard conditions at each altitude layer.
In the troposphere (0–11 km), temperature decreases by 6.5°C per 1000 m (the lapse rate). At the tropopause (~11 km), temperature stabilises at −56.5°C. This calculator uses ISA troposphere values only (valid to 11,000 m / 36,089 ft).
Applications include aviation (altimetry), meteorology, hiking and mountaineering planning, and engineering calculations involving air density.