Density Altitude
Pressure altitude and density altitude from the field elevation, altimeter setting and temperature, with an optional dew point and an optional live METAR fill, and what the result means for performance.
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Enter the field elevation, the altimeter setting (inches of mercury or hectopascals) and the outside air temperature (°C or °F). The page shows the pressure altitude, the density altitude and how far the density altitude is above the field. It starts with the example of the Pilot's Handbook of Aeronautical Knowledge: a field at 5,048 ft on a 30 °C day gives a density altitude of about 7,855 ft. To start from a real report, type an airport identifier and press Fill from METAR; the elevation, the altimeter setting, the temperature and the dew point are read from the latest METAR (the same data as the METAR Decoder) and you can still change every value. Without a connection, or for an unknown identifier, nothing is changed and the calculator works as before.
What density altitude tells you. The aircraft performs as if the airport were at the density altitude, whatever its real elevation: an airport at 500 ft with a density altitude of 5,000 ft performs like an airport at 5,000 ft. A high density altitude means thinner air, so the takeoff roll and the landing roll get longer, the rate of climb falls, the engine of a normally aspirated aircraft makes less power, and at the same indicated airspeed the true airspeed is higher. Hot, high and humid days make it worse. The published performance figures of the aircraft manual assume standard conditions at sea level, so always read the performance charts of the aircraft manual with the density altitude of the day (FAA Safety Team brochure on density altitude).
The dew point. Humidity is usually left out of the density altitude, because its main effect is on engine power. The optional dew point adds the density altitude of the moist air next to the standard value: water vapour is lighter than dry air, so humid air is a little thinner. The extra is largest on hot, humid days (a few hundred feet) and small when the air is dry. If humidity is high the FAA brochure also advises adding 10 percent to the computed takeoff distance and expecting a lower climb rate. The moist value is a model value for training; the standard value is the one performance charts use.
The words under the readout, from reduced to severe, compare the density altitude with the field elevation and are this tool's own wording, not FAA limits: they say how far the airplane is from its handbook figures, nothing more. Above 5,000 ft of density altitude lean a normally aspirated engine for takeoff.
Assumption: this is a training and planning aid, not for operational use. Real performance depends on the aircraft, its weight, the runway and the wind: use the aircraft manual charts and the weather report of the day. The METAR fill is for practice; get an official weather briefing before flight.
Pressure altitude = field elevation + the standard-atmosphere altitude of the altimeter setting (a setting of 29.92 inHg = 1013.25 hPa gives pressure altitude equal to the elevation; each 0.01 inHg is about 10 ft). It is the exact relation of the standard atmosphere, not the rule of thumb of 1,000 ft per inch of mercury. Pressure and density altitude are shown to the nearest 10 ft, because an altimeter setting is given to 0.01 inHg.
Density altitude is the altitude in the standard atmosphere (ICAO / U.S. Standard Atmosphere 1976: 15 °C and 1013.25 hPa at sea level, 6.5 °C per km) where the air has the same density as the actual air, found from the pressure at the pressure altitude and the outside air temperature (ideal gas law). It is the FAA definition, pressure altitude corrected for nonstandard temperature, solved exactly. The rule of thumb of 120 ft per °C above the standard temperature is shown for comparison; it is a little high.
The moist air. The saturation vapour pressure at the dew point comes from the Buck (1996) equation, checked against the IAPWS values to 0.1 %. With the vapour pressure e and the total pressure p, the density of the moist air is (p − e) ÷ (R × T) for the dry part plus e ÷ (Rv × T) for the vapour, which is the same as using the virtual temperature in the dry-air formula. The density altitude of the moist air is the standard-atmosphere altitude with that density.
Verified: the handbook example (5,048 ft: 7,855 ft at 30 °C and 1,232 ft at −25 °C) to within 6 ft, the pressure altitude table of the handbook within 2 ft, and the FAA density altitude rule-of-thumb chart (30 values) within 310 ft, which is the accuracy of that chart; the vapour pressures against the IAPWS table, and the moist air density against the ideal gas law written out separately.
The METAR fill uses the same data service and decoder as the METAR Decoder. It takes the temperature and dew point (to a tenth of a degree when the report has them), the altimeter setting in the unit the report uses and the station elevation, which the data service gives in metres. The report, its source and time are shown under the button; the elevation is the station elevation of the data service, so compare it with the airport chart.
- FAA Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25C (2023), ch. 8 Flight Instruments: pressure altitude and density altitude, with the 5,048 ft example (7,855 ft at 30 °C, 1,232 ft at −25 °C); ch. 11 Aircraft Performance: the standard atmosphere (Figure 11-2) and pressure altitude from the altimeter setting (Figure 11-3)
- FAA Safety Team brochure Density Altitude (P-8740-02, FAA Safety Team library): the definition, the effects of a high density altitude on takeoff distance, rate of climb, true airspeed and landing roll, leaning a normally aspirated engine above 5,000 ft of density altitude, the 10 percent advice for high humidity, and the Density Altitude Rule-of-Thumb chart used to check the values
- U.S. Standard Atmosphere, 1976 (NASA-TM-X-74335) / ICAO Doc 7488 — pressure, temperature and density against altitude in the standard atmosphere (the same model as the airspeed tool)
- Humidity: Buck (1996), New equations for computing vapor pressure and enhancement factor, Journal of Applied Meteorology 35 — saturation vapour pressure over water, checked against the IAPWS-95 values (Wagner and Pruss, 2002); the ideal gas law and Dalton’s law for moist air (virtual temperature, AMS Glossary of Meteorology)
- FAA-H-8083-28B Aviation Weather Handbook (2026), ch. 24 Aviation weather reports: the temperature and dew point group and the altimeter group of the METAR read by the optional fill
- NOAA / National Weather Service Aviation Weather Center Data API (aviationweather.gov/data/api, read 2026-10-23): the latest METAR for the optional fill, reached through this site’s relay; NWS information is in the public domain (weather.gov disclaimer)
- Model choices of this tool (stated on the page): the density altitude is the exact standard-atmosphere solution and the rule of thumb of 120 ft per °C is shown beside it; the moist-air value is a model value shown next to the standard one; the words describing the effect compare the density altitude with the field elevation and are not FAA limits