How to use it
- Type the Field elevation and choose its unit in Elevation is in.
- Enter the Altimeter setting (QNH) from the METAR and choose inHg or hPa in Altimeter setting is in.
- Enter the Air temperature (OAT). Add the Dew point from the METAR, or leave it empty for dry air.
- Optional: enter the Indicated airspeed (IAS) to get the True airspeed (kt).
- Read the Density altitude (ft), then open The working, step by step to see how it was reached.
How the density altitude is found
Density altitude is the altitude in the standard atmosphere at which the air has the same density as the air at the airfield. The FAA defines it as pressure altitude corrected for nonstandard temperature variations.
First the pressure altitude is found from the field elevation and the altimeter setting. Then the ISA deviation shows how far the air temperature is from the standard temperature at that altitude. Last, the density altitude is computed from the air density, which also depends on the water vapour and the pressure.
The FAA Instrument Flying Handbook gives a quick estimate for when no chart is at hand: add 120 feet for each degree Celsius above the ISA temperature. At 3,000 ft pressure altitude and 20 °C, the ISA temperature is about 9 °C, the difference is 11 °C, and the estimate is 3,000 + 120 × 11 = 4,320 ft (FAA-H-8083-15B, page 4-6). The tool shows this quick estimate beside the exact density altitude, which stays the primary result.
Station pressure: p = QNH × (1 − 0.0065 × h ÷ 288.15)^5.25588, h the field elevation in metres
Pressure altitude: PA = 44,330.77 × (1 − (p ÷ 1,013.25)^0.190263) metres
ISA deviation = air temperature − (15 − 0.0065 × PA), in °C
Density altitude: DA = 44,330.77 × (1 − σ^0.234969) metres, with σ = (p ÷ 1,013.25) × (288.15 ÷ Tv)- Near sea level the pressure falls about 27 ft per hPa. A QNH below the standard 1,013.25 hPa makes the pressure altitude higher than the field elevation, and a QNH above it makes the pressure altitude lower.
- Hot or high air is thin. When the temperature is above the ISA temperature, the density altitude is higher than the pressure altitude; when it is below, the density altitude is lower.
The virtual temperature correction and when it matters
Water vapour is lighter than the nitrogen and oxygen it displaces, so moist air is less dense than dry air at the same pressure and temperature. The tool allows for this with the virtual temperature, Tv = T ÷ (1 − 0.378 × e ÷ p), where e is the vapour pressure at the dew point. It is found from the Magnus form of the saturation vapour pressure over water (Alduchov and Eskridge, 1996).
The correction is small in cool weather and grows with heat. At 6,000 ft and 35 °C, a dew point of 20 °C raises the density altitude by about 350 ft. The FAA's Density Altitude brochure says humidity is not generally a major factor, because its main effect is on engine power, so treat the dew point as a refinement. Leave it empty to see the dry-air result.
What this calculator does not cover
- Aircraft performance. Take-off distance, climb rate and landing roll depend on the aircraft, its weight, flap setting, runway and engine. Read them from the charts in the aircraft's flight manual, using the density altitude as the input.
- Wind, runway slope, runway surface and the power setting you use.
- Airspeed instrument and position errors. The true airspeed here takes the indicated speed as the equivalent airspeed and ignores compressibility, which is adequate only at low speeds.
- Weather at the time of the flight. Use the current METAR or the airfield's observation, not a forecast.
- Real flight decisions. This is a tool for planning practice and training. Use the aircraft's flight manual and approved performance data for any real flight.
Where the figures come from
The standard atmosphere is the ICAO model: 15 °C and 1,013.25 hPa at sea level, a temperature lapse of 0.0065 K per metre (about 2 °C per 1,000 ft) up to the tropopause, and the exponent 5.25588 = g ÷ (R × L). The NASA Glenn atmosphere equation uses the same lapse rate and the exponent 5.256.
The unit factors are exact: 1 ft is 0.3048 m, and 1 inHg is 33.86389 hPa. The FAA's chapter 11 of the Pilot's Handbook gives the same standard: 29.92 inHg, or 1,013.2 mb, is the standard sea-level pressure. The 120 feet per degree Celsius quick estimate is from the FAA Instrument Flying Handbook, page 4-6.
Worked example: 1,500 ft, QNH 29.85 inHg, 30 °C, dew point 18 °C
The field elevation is 1,500 ft, which is 457.2 m. The altimeter setting of 29.85 inHg is 1,010.8 hPa, below the standard 1,013.25 hPa. The station pressure is 957.2 hPa.
The pressure altitude is 1,565 ft, 65 ft above the field. At that altitude the ISA temperature is 11.9 °C, so the 30 °C air is 18.1 °C above standard.
The dew point of 18 °C gives a vapour pressure of 20.6 hPa. The density altitude is therefore 3,904 ft, about 2,340 ft above the pressure altitude. With 110 kt indicated airspeed, the true airspeed is 117 kt.
The handbook's quick estimate, 1,565 ft plus 120 ft for each of the 18.1 °C above ISA, gives 3,737 ft, about 170 ft lower here.
| Result | Value |
|---|---|
| Density altitude | 3,904 ft |
| Pressure altitude | 1,565 ft |
| ISA temperature at pressure altitude | 11.9 °C |
| ISA deviation | 18.1 °C |
| Quick estimate, 120 ft per °C | 3,737 ft |
| True airspeed at 110 kt indicated | 117 kt |
Frequently asked questions
What is density altitude?
It is the altitude in the standard atmosphere at which the air has the same density as the air at the airfield. Hot or high air is less dense, so the density altitude is higher than the field elevation, and the aircraft needs a longer take-off roll and climbs more slowly.
Why is the pressure altitude different from the field elevation?
The altimeter setting moves it. When the QNH is below the standard 1,013.25 hPa, the pressure altitude is higher than the elevation; when it is above, the pressure altitude is lower. In the worked example, a QNH of 29.85 inHg puts it 65 ft above the field.
Do I need the dew point?
No, but it changes the answer a little. Without it the tool treats the air as dry. In the hot case of 35 °C at 6,000 ft, a dew point of 20 °C adds about 350 ft to the density altitude.
Can I use this figure to plan a take-off?
Use it only as the input to the performance charts in your aircraft's flight manual, which give take-off distance and climb for your weight and configuration. This tool is for planning practice and training, not for real flight.
What does the ISA deviation tell me?
It shows how far the outside air temperature is above or below the standard temperature for the pressure altitude. A large positive deviation means the air is thinner than the pressure altitude suggests, so the aircraft performs less well.
How accurate is the true airspeed?
It is an estimate. The tool takes the indicated speed as the equivalent airspeed and ignores compressibility, which is fine at low speeds. The FAA's rule of thumb of 2 percent per 1,000 ft agrees with the exact figure within about 3 percent up to 10,000 ft.
What is the 120 ft rule?
It is the handbook's quick estimate of density altitude when no chart is available: pressure altitude plus 120 ft for each degree Celsius above the ISA temperature. It is less accurate far from standard, so use the exact density altitude from this tool and the aircraft's charts for performance.
Sources
- FAA Pilot's Handbook of Aeronautical Knowledge, chapter 11: Aircraft Performance (pressure and density altitude)
- FAA Pilot's Handbook of Aeronautical Knowledge, chapter 8: Flight Instruments (true airspeed rule of thumb)
- FAA Density Altitude safety brochure (P-8740-02)
- NASA Glenn Research Center: the Earth atmosphere equation (English units)
- FAA Instrument Flying Handbook (FAA-H-8083-15B), page 4-6: density altitude estimated by adding 120 feet per degree Celsius above ISA
- Wikipedia: Vapour pressure of water (Magnus form with the Alduchov and Eskridge 1996 coefficients)
Figures and text last checked on Tuesday, November 10, 2026.