ISA Calculator

International Standard Atmosphere (ISA) Notes

• Based on the ICAO Standard Atmosphere (1993) which extends to 80km.
• The ISA is a static atmospheric model that defines how pressure, temperature, density, and viscosity change with altitude.
• The atmosphere is divided into layers with different temperature profiles:
  • Troposphere (0-11km): Temperature decreases linearly at 6.5°C/km from 15°C at sea level to -56.5°C at the tropopause.
  • Tropopause (11-20km): Constant temperature layer at -56.5°C.
  • Stratosphere I (20-32km): Temperature increases at 1.0°C/km.
  • Stratosphere II (32-47km): Temperature increases at 2.8°C/km up to -2.5°C.
  • Stratopause (47-51km): Constant temperature layer at -2.5°C.
  • Mesosphere I (51-71km): Temperature decreases at 2.8°C/km.
  • Mesosphere II (71-84.85km): Temperature decreases at 2.0°C/km to -86.2°C.
• Standard sea level conditions: 15°C (288.15K), 1013.25 hPa (101,325 Pa), and density of 1.225 kg/m³.
• The model assumes dry air, constant gravity, and linear temperature distributions within layers.
• Pressure and density are calculated using the hydrostatic equation and ideal gas law.
• This calculator is based on the mathematical model of the ISA and provides accurate values up to 86,000 meters.
• Note on temperature calculations: When calculating altitude from temperature, results are only reliable in the Troposphere (0-11km) because it has a consistent lapse rate. In isothermal layers (like the Tropopause) or where temperature values repeat at different altitudes (in Stratosphere/Mesosphere), a single temperature can correspond to multiple altitudes.