Standard Atmosphere (ISA)

The ICAO standard atmosphere is an agreed model: sea level at 15 °C and 1013.25 hPa, temperature falling 6.5 °C per kilometre to the tropopause at 11 km (36,089 ft) and then constant at −56.5 °C. Pressure follows from hydrostatic balance and the gas law: p = p₀ × (T ÷ T₀)^5.2559 in the troposphere and an exponential decay in the isothermal layer above. Density is p ÷ (R × T) and the speed of sound √(γRT). Altimeters, airspeed indicators and performance charts are all calibrated to it.

θ = T ÷ T₀, δ = p ÷ p₀ and σ = ρ ÷ ρ₀ are the temperature, pressure and density ratios that engine and aircraft performance charts are written in; √σ links true and equivalent airspeed, and δ is what the pressure altitude scale encodes.

The real atmosphere is rarely standard: the ISA deviation of an outside air temperature — OAT minus the standard temperature at that altitude — is the figure that drives density altitude and true airspeed. The page gives it when you enter an OAT.

Geopotential altitude to 20 km only; humidity, non-standard lapse rates and the actual pressure field are not modelled beyond the temperature deviation. A planning aid, not a substitute for the pilot’s operating handbook, the aircraft’s limitations or the rules that apply to the flight. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.