What Is Density Altitude?
Density altitude is the pressure altitude corrected for temperature deviation from the International Standard Atmosphere (ISA). It represents the altitude in the standard atmosphere at which the actual air density would be found. Aircraft performance (engine power, propeller efficiency and wing lift) depends on air density, not just physical height above the ground. So two airfields at the same elevation can have very different aircraft performance if one is hot and the other is cold: the hot airfield behaves as though it is at a much higher altitude.
How This Calculator Works
The calculation happens in two steps, using the standard formulas taught in pilot training and used in aviation performance charts worldwide.
| Step | Formula |
|---|
| 1. Pressure altitude | Pressure altitude (ft) = Field elevation (ft) + [(1013.25 - QNH in hPa) × 27] |
| 2. ISA standard temperature | ISA temp (°C) = 15 - (1.98 × Pressure altitude / 1000) |
| 3. Density altitude | Density altitude (ft) = Pressure altitude + [120 × (Actual OAT - ISA temp)] |
The 27 feet per hectopascal figure comes from the standard atmosphere's rate of pressure change near sea level (equivalent to 1,000 feet per inHg of altimeter deviation from 29.92 inHg / 1013.25 hPa). The 1.98°C per 1,000 feet is the ISA standard lapse rate. The 120 feet per degree Celsius factor for the final density altitude adjustment is the standard approximation used throughout aviation, including by the US FAA and NZ CAA performance planning material.
Worked Example
Auckland Airport sits at 358 feet elevation. On a 20°C day with QNH at 1013.25 hPa (standard pressure, so pressure altitude equals field elevation): the ISA standard temperature at 358 feet is 15 - (1.98 × 0.358) = 14.29°C. The actual temperature of 20°C is 5.71°C above standard. Density altitude = 358 + (120 × 5.71) = 358 + 685 = 1,043 feet. So on this ordinary summer day, the aircraft performs as though the airfield were at roughly 1,043 feet rather than its actual 358 feet.
Why It Matters for Pilots
High density altitude reduces the power an engine can produce, reduces propeller thrust, and reduces the lift a wing generates at a given airspeed. The practical effects are longer takeoff rolls, reduced climb rate, higher true airspeed for the same indicated airspeed, and reduced service ceiling. In New Zealand, this matters most at high-elevation airfields such as Queenstown and Wanaka, especially on warm summer afternoons when density altitude can be 2,000 to 3,000 feet above field elevation. Always cross-check density altitude against your aircraft's performance charts before flight, particularly with a full load, a short runway, or high terrain nearby.
Density Altitude Categories
| Density Altitude Above Field Elevation | Performance Impact |
|---|
| Below 1,000 ft | Minimal effect on normal aircraft performance |
| 1,000 to 3,000 ft | Noticeable reduction in climb performance and takeoff distance margin |
| 3,000 to 5,000 ft | Significant performance degradation; check charts carefully, especially at high weight |
| Above 5,000 ft | Severe impact; many light aircraft see large increases in takeoff and landing distance |
Related Calculators
Sources: ICAO Standard Atmosphere (Doc 7488). US FAA Pilot's Handbook of Aeronautical Knowledge, density altitude section. NZ Civil Aviation Authority Good Aviation Practice guide, aircraft performance.
This calculator uses the standard approximation formulas for pressure altitude and density altitude used throughout general aviation training and planning. Results are indicative only. Always use your aircraft's official performance charts and current, certified weather data (METAR/QNH) for actual flight planning.
