Aircraft cabin pressurization: modern airliners hold cabin pressure at an equivalent altitude of 6,000–8,000 ft even while cruising far higher, which is why the cabin still feels breathable.
Temperature: The standard atmosphere temperature at your altitude. Actual conditions vary with weather, season, and latitude, but this gives the baseline pilots and engineers use for performance calculations.
Pressure (inHg / hPa): Atmospheric pressure drops roughly 1 inHg per 1,000 ft in the lower atmosphere. This affects engine performance, aircraft lift, and boiling points.
Air density: Lower density means less oxygen per breath and less aerodynamic lift. At 10,000 ft, density is about 74% of sea level.
Effective oxygen: The percentage of sea-level oxygen available. Below 90% (roughly above 6,000 ft), most people begin to notice reduced exercise capacity. Below 60% (above ~15,000 ft), supplemental oxygen is generally recommended.
Boiling point: Water boils at lower temperatures at altitude — roughly 1°C less per 300 m (1,000 ft) gain. This affects cooking times and sterilization procedures.
How This Calculator Works
You enter an altitude in feet or meters. The tool applies the International Standard Atmosphere (ISA) model: below 36,089 ft it uses the troposphere lapse rate of −1.981 °C per 1,000 ft and the barometric formula with exponent 5.256. Above that threshold it switches to the isothermal stratosphere model at −56.5 °C. Air density is derived from pressure and temperature using the ideal gas law. The calculator assumes standard conditions with no wind, humidity, or weather corrections.
Quick Questions
What is the International Standard Atmosphere?
The ISA is a reference model defined by ICAO that describes how pressure, temperature, and density change with altitude under idealized conditions. It's the baseline used worldwide for aircraft performance charts, altimeter calibration, and engineering calculations.
Why does actual weather differ from these numbers?
The ISA assumes a fixed lapse rate, no humidity, and no weather systems. Real conditions include temperature inversions, fronts, and moisture that shift pressure and density. Pilots use altimeter settings (QNH) to correct for these differences.
At what altitude do I need supplemental oxygen?
FAA regulations require supplemental oxygen for flight crews above 12,500 ft for more than 30 minutes, and for all occupants above 15,000 ft. For hikers and climbers, most people feel noticeable effects above 8,000 ft, and acclimatization becomes critical above 10,000 ft.
Does this work for negative altitudes (below sea level)?
Yes — entering a negative value gives pressure higher than sea level. The Dead Sea (~−1,400 ft) has roughly 5% higher atmospheric pressure than sea level, which the ISA formula handles correctly in the troposphere range.