How to use
- Choose feet and inHg or metres and hPa.
- Enter the field elevation and the altimeter setting (QNH) from the ATIS or METAR.
- Choose °C or °F and enter the outside air temperature.
- Read the density altitude, the pressure altitude and how far the temperature is above or below standard.
Formulas
- pressure altitude (ft) = elevation + 1000 × (29.92 − altimeter inHg) = elevation + (1013.25 − QNH hPa) × 1000 / 33.8639
- ISA temperature (°C) = 15 − 1.98 × pressure altitude / 1000
- density altitude (ft) ≈ pressure altitude + 120 × (OAT − ISA temperature)
Second figure: station pressure P = QNH × (1 − 6.8755856·10⁻⁶ × elevation ft)^5.2558797, dry-air density ρ = P / (287.05 × T in kelvin), density altitude = 145442.16 × (1 − (ρ / 1.225)^0.234969) ft.
Worked example
Elevation 5,000 ft, altimeter 29.92 inHg, 30 °C (the default values):
- pressure altitude = 5,000 + 1000 × (29.92 − 29.92) = 5,000 ft;
- ISA temperature = 15 − 1.98 × 5 = 5.1 °C, so the air is 24.9 °C warmer than standard;
- density altitude = 5,000 + 120 × 24.9 = 7,988 ft (2,435 m).
The density-based figure is 7,802 ft. Either way, the aircraft performs as if it were close to 8,000 ft.
Tips
- High, hot and humid is the dangerous combination: humid air is lighter still, so treat the result as the best case on a humid day.
- Density altitude changes during the day; the cool early morning often gives far better performance than the afternoon.
- Set the altimeter to 29.92 inHg (1013 hPa) to read pressure altitude directly on the instrument.