About the Density Altitude Calculator
This density altitude calculator works out the altitude in the standard atmosphere that has the same air density as the conditions at your airfield right now. Enter the field elevation, outside air temperature, altimeter setting and (optionally) the dew point, and it returns density altitude in feet and metres, pressure altitude, station pressure, the ISA standard temperature for comparison, and the actual air density.
Pilots use density altitude because aircraft performance — engine power, propeller thrust and wing lift — depends on air density, not on the altitude shown on a chart. On a hot, high or humid day the density altitude can be thousands of feet above the field elevation, lengthening the take-off roll and reducing the climb rate. Drone operators, skydivers, racers tuning engines and ballistics shooters use the same number.
The calculation follows the National Weather Service method: station pressure from the altimeter setting and elevation, virtual temperature to account for humidity, then density altitude from the standard-atmosphere relation. A rule-of-thumb value (pressure altitude + 120 ft per °C above standard) is shown for comparison. Always use your aircraft’s Pilot’s Operating Handbook for actual performance figures.
With the default inputs, the density altitude is 7,997 ft. Change any value above to recalculate instantly.
How to use the density altitude calculator
- 1Enter the field elevation and choose feet or metres.
- 2Enter the outside air temperature from the METAR, ATIS or a thermometer.
- 3Enter the altimeter setting in inHg or hPa.
- 4Leave humidity on and enter the dew point for the most accurate result.
- 5Read the density altitude and compare it with your POH performance charts.
Formula and method
First the altimeter setting is converted to the actual station pressure at the field elevation h (in metres). The dew point gives the water-vapour pressure e (hPa), and the virtual temperature Tv is the temperature dry air would need to have the same density as the moist air — humid air is lighter than dry air because water vapour (molar mass 18) is lighter than nitrogen and oxygen (about 29). Density altitude is then found from the standard-atmosphere relation using station pressure in inches of mercury and virtual temperature in degrees Rankine (K × 1.8). These are the formulas published by the US National Weather Service.
Pressure altitude uses PA = 145,366.45 × (1 − (P_stn/1013.25)^0.190284). The ISA standard temperature at that altitude is 15 °C minus 1.98 °C per 1,000 ft. Air density is ρ = P / (287.05 × Tv). The rule of thumb used in flight training, DA ≈ PA + 120 ft × (OAT − ISA), is shown for comparison and agrees closely in dry air.
- P_alt
- Altimeter setting (QNH)
- P_stn
- Station pressure at field elevation
- h
- Field elevation (m)
- Td
- Dew point (°C)
- e
- Water-vapour pressure (hPa)
- Tv
- Virtual temperature (K, then °R)
- DA
- Density altitude (ft)
Worked examples
5,000 ft field on a 30 °C day, dew point 10 °C
Station pressure is 29.92 × (1 − 0.0065 × 1524/288)^5.2561 = 24.89 inHg (842.9 hPa). With 12.3 hPa of water vapour the virtual temperature is 304.8 K, giving a density altitude of about 7,997 ft — nearly 3,000 ft above the field.
Same day in dry air
Without humidity the density altitude is about 7,818 ft, so moisture adds roughly 180 ft here. The rule of thumb — 5,002 ft + 120 × (30 − 5.1) — gives about 7,991 ft, close to the full calculation.
Metric: 1,600 m elevation, 35 °C, QNH 1020 hPa, dew point 15 °C
High pressure helps a little, but the heat dominates: station pressure is 840.7 hPa and the density altitude is about 8,680 ft (2,645 m) — over 3,400 ft above the 5,249 ft field.
Frequently asked questions
What is density altitude?+
Density altitude is pressure altitude corrected for non-standard temperature (and humidity). It is the height in the International Standard Atmosphere at which the air would have the same density as the air where you are.
Why does density altitude matter for pilots?+
Thinner air reduces engine power, propeller thrust and wing lift. At high density altitude an aircraft needs a longer runway, climbs more slowly and has a higher true airspeed for the same indicated airspeed.
What is the difference between pressure altitude and density altitude?+
Pressure altitude is the altitude shown when the altimeter is set to 29.92 inHg (1013.25 hPa). Density altitude adds a correction for temperature and humidity. They are equal only in standard conditions.
Does humidity increase density altitude?+
Yes. Water vapour is lighter than dry air, so humid air is less dense. The effect is usually a few hundred feet — smaller than temperature, but worth including on hot, humid days.
What is a quick rule of thumb for density altitude?+
Add about 120 ft to the pressure altitude for every 1 °C the outside temperature is above the ISA standard (15 °C at sea level, falling about 2 °C per 1,000 ft). This calculator shows that estimate alongside the full result.