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.
Why there is no aircraft picker here. Density altitude depends on the airfield and the weather, not on the aircraft parked at it. The same figure applies to everything on the ramp.
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.
Density altitude by elevation and temperature
Density altitude is what the aircraft's performance responds to, and on a hot day it is thousands of feet above the airfield elevation. Each cell is the density altitude at the elevation on the left and the temperature along the top, with the altimeter set to 1,013 hPa.
| Airfield elevation | 10 °C | 20 °C | 30 °C | 40 °C |
|---|---|---|---|---|
| 0 ft | -591 ft | 609 ft | 1,809 ft | 3,009 ft |
| 1,000 ft | 647 ft | 1,847 ft | 3,047 ft | 4,247 ft |
| 2,000 ft | 1,884 ft | 3,084 ft | 4,284 ft | 5,484 ft |
| 3,000 ft | 3,122 ft | 4,322 ft | 5,522 ft | 6,722 ft |
| 5,000 ft | 5,597 ft | 6,797 ft | 7,997 ft | 9,197 ft |
| 7,000 ft | 8,072 ft | 9,272 ft | 10,472 ft | 11,672 ft |
What the table holds fixed
- Altimeter setting held at 1,013 hPa, so the pressure altitude equals the elevation and the whole effect in the table is temperature.
- This is the standard planning approximation, 120 feet per degree above ISA. It is not a substitute for the flight manual chart, which is what the approved performance figures are read from.
- Humidity reduces density further and is not modelled here. On a hot, humid day treat the figure as optimistic.
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.