ISA Temperature Calculator

Summary

Enter an altitude to instantly get ISA standard temperature, pressure, density, and speed of sound using the ICAO standard atmosphere model.

How it works

1. Enter your altitude in feet or meters using the unit toggle.
2. The calculator applies the ICAO two-layer standard atmosphere: troposphere (0–11 km) uses a 6.5°C/km temperature lapse rate; lower stratosphere (11–20 km) is isothermal at −56.5°C.
3. Temperature is derived from the lapse rate formula: T = T₀ − L × h (troposphere) or T = −56.5°C (stratosphere).
4. Pressure is computed from the hypsometric equation for the troposphere and the isothermal layer equation for the stratosphere.
5. Air density follows from the ideal gas law: ρ = P / (R × T), where R = 287.05 J/(kg·K).
6. Speed of sound is calculated as a = √(γ × R × T), with γ = 1.4 for dry air.
7. Results include the ISA deviation when you enter an actual observed temperature alongside the standard values.

Use cases

• Determine the standard temperature, pressure, and density at cruise altitude for aircraft performance calculations.
• Compute the ISA deviation (OAT minus ISA standard temperature) for a given flight level.
• Verify altimeter settings and pressure altitude references against the standard atmosphere.
• Support aerodynamic and structural analysis by providing standard air density at design altitude.
• Assist student pilots and engineers in understanding the standard atmosphere layers and lapse rates.
• Cross-check avionics and flight management system atmospheric data against known ISA values.
• Plan high-altitude balloon or UAV missions by reviewing density at target altitude.
• Validate simulation software outputs against the ICAO standard atmosphere.

Frequently Asked Questions