ISA Calculator
Standard-atmosphere temperature, pressure and density at any altitude.
Inputs
Result
ISA temperature
-4.8°C
Standard temp at 10,000 ft
ISA deviation
Colder than standard
-0.2°C
Standard pressure
696.8hPa
Density ratio
ρ / ρ₀
0.738
ISA temp -4.8 °C at 10,000 ft (deviation -0.2 °C)
Formula
Ref- ISA temp = 15 − 1.98 × (altitude ÷ 1000) [°C]
- ISA deviation = actual OAT − ISA temp
- Pressure & density ratios from the ISA model
Worked example
RefAltitude 10,000 ft, actual OAT −5 °C.
- 1.ISA temp = 15 − 1.98 × 10 ≈ −4.8 °C.
- 2.Actual −5 °C vs ISA −4.8 °C.
- 3.ISA deviation ≈ −0.2 °C (near standard).
ISA temp ≈ −4.8 °C at 10,000 ft; deviation ≈ −0.2 °C.
How it works
RefThe International Standard Atmosphere is the agreed reference every performance chart, altimeter and airspeed correction is built on: 15 °C and 1013.25 hPa at sea level, cooling 1.98 °C per 1,000 ft. Comparing the actual temperature to the standard gives the ISA deviation that drives true altitude and true airspeed corrections.
DGCA student tip
ExamThe taught lapse rate is 2 °C per 1,000 ft (the precise figure is 1.98). Standard temperature at a level = 15 − 2 × (altitude in thousands). A positive ISA deviation means warmer, thinner air and higher true altitude.
Common mistakes
Watch- Using 3 °C/1,000 ft (the dry adiabatic rate) instead of the 2 °C ISA lapse.
- Forgetting the lapse stops at the tropopause (~36,000 ft).
- Confusing ISA deviation with the temperature itself.