What this calculator does
Density altitude is the altitude at which the air density would match current conditions, if the atmosphere were at standard temperature. It is the figure that actually determines aircraft and engine performance, since a hot day at a low-elevation airport can behave, aerodynamically, like a much higher airfield on a standard day: longer takeoff rolls, reduced climb performance and a higher true airspeed for the same indicated airspeed.
This calculator uses a widely taught rule of thumb rather than the full density altitude formula involving air pressure directly. It starts from the standard-atmosphere temperature expected at a given pressure altitude, then adjusts for how far the actual outside air temperature departs from that standard, which is normally the dominant factor in day-to-day density altitude changes at a given airport.
The formula
Work out the ISA standard temperature at the pressure altitude entered (15°C minus 2°C for every 1,000 feet). Density altitude is then the pressure altitude, plus 120 feet for every degree Celsius the actual outside air temperature sits above that ISA value (or minus, if colder than ISA).
| Term | Meaning |
|---|---|
| Pressure altitude | The altitude shown when the altimeter is set to 29.92 in Hg (1013.25 hPa), removing the effect of local pressure. |
| ISA temperature | The standard-atmosphere temperature expected at a given altitude: 15°C at sea level, falling by 2°C per 1,000 feet. |
| Density altitude | The altitude that matches the actual air density, accounting for how far actual temperature departs from ISA at that pressure altitude. |
The inputs explained
| Field | What to enter |
|---|---|
| Pressure altitude (ft) | The pressure altitude for the airport or location, found by setting the altimeter to 29.92 in Hg (1013.25 hPa) and reading the indicated altitude, or from a current pressure altitude report. |
| Outside air temperature (OAT) (°C) | The actual outside air temperature at that location, in degrees Celsius. |
When to use it
Checking takeoff performance on a hot day
A high density altitude at a hot, high-elevation airport is one of the classic causes of longer-than-expected takeoff rolls, and this gives a fast estimate before checking the aircraft’s actual performance charts.
Briefing density altitude for the day
Working this out at the start of a flying day, using the current temperature and field pressure altitude, gives a single figure to keep in mind for performance planning.
Understanding why the same airport performs differently across seasons
The same field elevation produces a much higher density altitude, and correspondingly worse performance, on a hot summer afternoon than on a cold winter morning.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How density altitude changes with outside air temperature, at a fixed pressure altitude
A fixed 3,000 ft pressure altitude, across a range of outside air temperatures.
| Outside air temperature | Density altitude | Temperature above ISA |
|---|---|---|
| 0°C | 1,920 ft | -9.0°C |
| 10°C | 3,120 ft | 1.0°C |
| 20°C | 4,320 ft | 11.0°C |
| 30°C | 5,520 ft | 21.0°C |
| 35°C | 6,120 ft | 26.0°C |
| 40°C | 6,720 ft | 31.0°C |
How density altitude changes with pressure altitude, at a fixed outside air temperature
A fixed 30°C temperature, across a range of pressure altitudes.
| Pressure altitude | Density altitude | ISA temperature at this altitude |
|---|---|---|
| 0 ft | 1,800 ft | 15.0°C |
| 1,000 ft | 3,040 ft | 13.0°C |
| 2,000 ft | 4,280 ft | 11.0°C |
| 3,000 ft | 5,520 ft | 9.0°C |
| 5,000 ft | 8,000 ft | 5.0°C |
| 7,000 ft | 10,480 ft | 1.0°C |
Questions
Why does density altitude matter more than field elevation?
Because engine power, propeller efficiency and wing lift all depend on actual air density rather than altitude as such. Two airfields at the same elevation can have very different performance on a given day if their temperatures differ, which is exactly what density altitude captures.
Is this rule of thumb accurate enough for real flight planning?
It gives a reasonable planning estimate but is not a substitute for reading density altitude directly off a proper performance chart or flight computer when takeoff or landing performance is genuinely marginal.
Does humidity affect density altitude?
Yes, humid air is slightly less dense than dry air at the same temperature and pressure, which pushes density altitude a little higher still. This rule of thumb, like most quick estimates, does not account for humidity.
How do I get pressure altitude if I only have field elevation and the current altimeter setting?
Pressure altitude is approximately field elevation plus 1,000 feet for every 1 in Hg the altimeter setting is below 29.92 (or minus, if above), or read directly by setting 29.92 in Hg on the altimeter and noting the indicated altitude.
For how that same temperature effect shows up in indicated versus true airspeed, see the true airspeed estimate calculator. For fuel and range planning for the flight itself, see the fuel burn endurance and range calculator.