Standard Atmosphere (ISA) Calculator

The International Standard Atmosphere (ISA) is a reference model — not a forecast — that defines a single idealized profile of temperature, pressure, and density against altitude, used as the baseline every altimeter, performance chart, and airspeed indicator is calibrated against. This calculator reproduces the sea-level reference values and troposphere lapse-rate equation exactly as published in ICAO Doc 7488, Manual of the ICAO Standard Atmosphere — the same international standard also issued as ISO 2533:1975 — rather than an approximation: T₀ = 288.15 K, P₀ = 101,325 Pa, and a lapse rate of 0.0065 K/m (6.5°C per km) up to the 11,000 m (36,089 ft) tropopause. Below 36,089 ft (the tropopause), ISA temperature falls at a constant lapse rate of about 1.98°C per 1,000 ft as altitude increases, approximating the radiative-convective equilibrium of rising air expanding and cooling adiabatically as atmospheric pressure drops — not a change in the air's own density or in greenhouse trapping. Pressure falls with altitude too, following the barometric formula built from that same temperature profile, and pressure actually falls the fastest of the three ISA quantities: air density falls more slowly than pressure because the accompanying temperature drop partly offsets it (colder air packs more molecules into the same volume at a given pressure), while temperature itself falls the least steeply of the three, in percentage terms, through the troposphere.

Above the tropopause, ISA temperature holds constant at -56.5°C through the lower stratosphere, and pressure and density continue falling on a pure exponential decay rather than the lapse-rate-driven curve used below it. Density ratio (σ) and pressure ratio (δ) — both expressed relative to sea-level standard values — are the terms pilots actually use in performance calculations like true airspeed and engine power correction. What this tool does not account for: real-world weather, which routinely differs from ISA by 10-20°C or more; this is a reference model for comparison and calibration, not today's actual atmosphere at your location.