Density Altitude
Density altitude from pressure altitude (or elevation and QNH) and the OAT, exact through the ISA density law, with the 120 ft per °C rule beside it.
The density altitude and how far above the field it is, the pressure altitude and ISA deviation it rests on, the density ratio and air density, and the rule-of-thumb figure with its difference from the exact one.
Example: A 5,000 ft field, 29.92 inHg and 30 °C: ISA there is 5.1 °C, so 24.9 °C warm; density ratio 0.791 gives a density altitude of 7,795 ft — the rule of thumb says 7,990.
Thin air has
an altitude.
What density altitude is and why performance follows it, how the exact figure is derived from pressure and temperature, where the rule of thumb sits, and what humidity does.
From pressure and temperature
Air density is pressure over temperature: ρ = p ÷ (R × T). The pressure is the station pressure — the ISA pressure at the pressure altitude, which the page finds from elevation and QNH or takes as entered — and the temperature is the outside air temperature you read. Density altitude is the height in the standard atmosphere that has that density, found from the ISA density law; it is what the engine, propeller and wing feel.
The rule of thumb
ISA temperature falls about 2 °C per 1,000 ft, and density altitude rises about 120 ft for every degree the air is warmer than ISA, so DA ≈ PA + 120 × (OAT − ISA) is the working rule. It is close at low altitude and mild deviations; the exact figure is shown beside it with the difference.
Humidity and performance
Water vapour is lighter than dry air, so humid air is a little less dense and the true density altitude a little higher than the dry figure here — a few hundred feet at most on a hot humid day. What a given density altitude does to take-off roll, climb rate and true airspeed is the aircraft’s own figure, from its handbook charts, at the density altitude you now have.
What the number does not settle
Density altitude is the input to performance charts, not the performance itself; the charts, weight and runway are yours. A planning aid, not a substitute for the pilot’s operating handbook, the aircraft’s limitations or the rules that apply to the flight. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.
SOURCES
- ICAO Doc 7488, Manual of the ICAO Standard Atmosphere (constants and layer laws)
Last reviewed 23 September 2026. How results are checked: How we verify.