Standard Atmosphere (ISA) Calculator
Look up ISA standard atmosphere values at any altitude: temperature, pressure, density, density ratio, and speed of sound.
Inputs
ft
Troposphere: 0–36,089 ft, Stratosphere: 36,089–65,617 ft
Results
ISA Temperature
74.05°F
Pressure
696.8 hPa
ISA Temperature23.3°F
Pressure20.58 inHg
Density Ratio (σ)0.7385
Air Density0.9046 kg/m³
Speed of Sound638 kt
Pressure Ratio (δ)0.6877
Temperature Ratio (θ)0.9312
How to Use This Calculator
- Enter any altitude to see ISA standard atmosphere properties.
- Use ISA temperature to calculate ISA deviation (OAT − ISA temp).
- Density ratio is used in airspeed conversions (TAS = CAS ÷ √σ).
How the result changes with Altitude
| Altitude | ISA Temperature | Pressure |
|---|---|---|
| 0 | 15°C | 1,013.3 hPa |
| 18,000 | -20.7°C | 506 hPa |
| 35,000 | -54.3°C | 238.4 hPa |
| 36,089 | -56.5°C | 226.3 hPa |
What each input means
- Altitude
- Altitude above mean sea level in feet.
What each result means
- Density Ratio (σ)
- Ratio of density to sea-level standard.
How this is calculated
Worked example, using the default values
- ISA TemperatureT = 15°C − 2°C × (Alt ÷ 1000 ft)15 − 2 × (10,000 ÷ 1000) = -4.8°C (23.3°F)
- Atmospheric PressureP = P₀ × (T/T₀)^(g/LR)101325 × (268.3 / 288.15)^5.256 = 696.8 hPa (20.58 inHg)
- Air Densityρ = P / (R × T)69682 / (287.05 × 268.3) = 0.9046 kg/m³ (σ = 0.7385)
- Speed of Sounda = 38.967 × √(T in Kelvin)38.967 × √268.3 = 638 kt
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