On a hot day at altitude the air is thinner than the altimeter suggests, and the aircraft performs as if the airport sat far higher. Density altitude is the number that captures this — and it sits behind a large share of summer takeoff accidents. This calculator estimates it from field elevation, pressure setting and temperature.
How this calculator works
Uses pressure altitude plus 120 feet for each degree Celsius above ISA temperature.
Worked example
An airport at 1,500 ft with QNH 1013 and +30 °C has a pressure altitude of about 1,500 ft, where the standard atmosphere would be +12 °C. The 18-degree excess adds roughly 2,200 ft, for a density altitude near 3,700 ft: the aircraft takes off as if the runway sat at 3,700 ft on a standard day.
Frequently asked questions
What is density altitude in plain terms?
The altitude the aircraft thinks it is at. Hot, thin air reduces engine power, propeller thrust and wing lift at the same time, so takeoff rolls stretch and climb rates sag. A sea-level pilot visiting a warm 2,000 ft airfield in July may effectively be operating from a 4,500 ft strip.
When does density altitude become dangerous?
Whenever the runway is short, the aircraft is heavy and the day is hot — the classic high, hot and heavy combination. A naturally aspirated piston loses roughly 3% of its power per 1,000 ft of density altitude, so at 6,000 ft it is down nearly a fifth of its rated power while the runway requirement grows.
What rule of thumb does this calculator use?
Pressure altitude plus 120 ft for every degree Celsius above the ISA temperature for that altitude — the same approximation taught in most training syllabi. It ignores humidity, which thins the air further, so on humid summer days treat the result as slightly optimistic.
What to verify next
- Approved aircraft documentation and current limitations.
- Local fuel, insurance, maintenance, hangar and financing quotations.
- The exact aircraft configuration, loading and maintenance status.