Density Altitude Calculator
Density altitude from field elevation, altimeter setting (hPa or inHg), temperature and dew point, by the full virtual-temperature method, with the 120 ft per °C rule of thumb shown beside it for comparison. Educational.
Density Altitude Calculator
Pre-filled with illustrative values for a hot, humid afternoon at a sea-level field in Singapore, not an observation. Singapore's and Malaysia's aeronautical information publications give the altimeter setting in hectopascals; inches of mercury are accepted too.
A flight-training shortcut, shown for comparison only. It is not stated in the FAA's density altitude pamphlet. It leaves out water vapour, and in hot air its 120 ft per degree is larger than the standard atmosphere's own factor, so the two errors pull in opposite directions.
How to use the density altitude calculator
Enter the field elevation
Type the aerodrome elevation in feet or metres; switch the unit beside the box to match. Elevations from −1,500 to 18,000 ft are accepted.
Enter the altimeter setting
Use QNH, the setting that makes an altimeter read the field elevation on the ground, in hectopascals or inches of mercury. If the number looks like the other unit, the calculator says so rather than guessing.
Enter temperature and dew point
Type the air temperature and the dew point, both in °C or both in °F. Aviation weather reports list the two side by side. If you only have relative humidity, switch the humidity box to it. Leave it blank and the result is a dry-air figure, labelled as such.
Read the result
The headline is the full calculation with water vapour included. Beside it you see the dry-air figure, how much humidity added, the pressure altitude and how far the air is from standard temperature. The 120 ft per °C rule of thumb sits below as a comparison line, with its gap from the full figure.
Density altitude from pressure, heat and water vapour
What the number describes
Density altitude is the height in the standard atmosphere at which the air would be as dense as the air you actually have. Lower pressure, higher temperature and more water vapour all make air less dense, so all three push it up. The FAA's density altitude pamphlet (P-8740-2) defines it as "pressure altitude corrected for nonstandard temperature variations", and warns that hot, high and humid weather "can cause a routine takeoff or landing to become an accident in less time than it takes to tell about it." The number describes the air. What a particular aircraft does in that air comes from its own flight manual, not from this page.
How the calculator works it out
It reduces the altimeter setting to the pressure at the field along the standard atmosphere, and reads the pressure altitude from that pressure. It then follows the method the US National Weather Service publishes with its own density altitude calculator: vapour pressure from the dew point, then the virtual temperature, the temperature dry air would need to be as light as the moist air you have. Finally it finds the standard-atmosphere height with that density. The standard values assumed are 15 °C and 1013.25 hPa at sea level, cooling 6.5 °C per kilometre.
"A wing has no idea how high the runway is. It meets only the air, and heat and water vapour both thin it."
Humidity in Singapore's air
The Meteorological Service Singapore publishes 1991–2020 normals for its Changi climate station. April has the hottest mean daily maximum, 32.4 °C, and its 24-hour mean temperature and humidity (28.2 °C, 82.6%) put the dew point near 25 °C by our conversion. At standard pressure and Changi's 22 ft elevation, the calculator puts the dry-air density altitude at 2,017 ft and the full figure at 2,416 ft: water vapour adds 399 ft. Running every month the same way, humidity adds between 373 and 407 ft. These are monthly averages, not readings from any single afternoon.
The dew point, not the air temperature, sets that amount. At sea level with the dew point held at 24 °C, moving the air temperature anywhere from 26 °C to 36 °C changes the humidity term by 3 ft or less in our calculation. A paper by Guinn and Barry (2016) makes the same point in its abstract: for a given pressure altitude, the humidity correction depends on the dew point alone.
The 120 ft per degree rule of thumb
Flight training teaches a shortcut: density altitude is pressure altitude plus 120 ft for every degree Celsius above standard temperature. It is a training rule; the FAA pamphlet prints a chart instead and does not state the figure. In our calculation, the standard atmosphere's own dry-air factor is 118.6 ft per °C at every altitude when the air is at standard temperature, and it shrinks as the air warms: averaged over a deviation of 10, 20 and 30 °C at sea level it is 116.1, 113.8 and 111.5 ft per °C. So in tropical heat the shortcut reads a little high for dry air, about 98 ft high at Changi's April normals, while ignoring the 399 ft that water vapour adds. Together it lands 301 ft below the full figure. Across all twelve Changi months it reads between 283 and 310 ft low. On a dry day at a 5,000 ft field at 35 °C and standard pressure, the full dry calculation puts density altitude at 8,329 ft and the rule of thumb at 8,589 ft.
Pressure moves it less than heat
At Changi's April temperature and dew point, each hectopascal of QNH moves density altitude by about 34 ft in our calculation. A pressure 7 hPa below standard therefore shifts the result by roughly what 2 °C of extra heat does.
Dew point, hectopascals and the 120-foot shortcut
Density altitude is the standard-atmosphere height with the same air density as the air you have.
The FAA pamphlet sets pressure altitude at 29.92 inHg, "1013 hPa in other parts of the world".
Singapore's and Malaysia's AIPs publish the altimeter setting in hectopascals.
At Changi's April normals, water vapour adds about 400 ft (our calculation).
Hold the dew point and the humidity term barely moves with air temperature: 3 ft or less from 26 to 36 °C (our calculation).
At standard temperature the dry factor is 118.6 ft per °C at every altitude (our calculation).
In warm air the factor shrinks, to 111.5 ft per °C averaged over 30 °C above standard at sea level (our calculation).
"120 ft per °C" is a training shortcut; the FAA pamphlet does not state it.
Near Changi, each hectopascal of QNH is worth about 34 ft (our calculation).
The FAA pamphlet treats humidity mainly as an engine-power effect rather than an aerodynamic one.
Frequently asked questions
It is the altitude in the standard atmosphere at which the air density matches the density you actually have. It rises with lower pressure, higher temperature and more water vapour, so a hot, humid day puts it well above the field elevation even at a sea-level airport. It describes the air; how a given aircraft performs in that air is set out in that aircraft's flight manual.
It turns the altimeter setting into the pressure at the field, using the standard atmosphere, and reads the pressure altitude from it. With a dew point (or relative humidity) it computes the vapour pressure and the virtual temperature, as the US National Weather Service's density altitude formula does, and then finds the standard-atmosphere height with the same density. Without humidity it does the same with dry air and labels the result. The 120 ft per °C rule of thumb is computed separately and shown underneath for comparison.
The FAA pamphlet describes pressure altitude as the indicated altitude with the altimeter set to 29.92 inHg, "1013 hPa in other parts of the world". It accounts for pressure only. Density altitude also accounts for temperature and, in this calculator, water vapour. When the air is dry and at standard temperature the two are the same; on a hot day density altitude is higher.
Yes. Water vapour is lighter than dry air, so humid air is less dense at the same temperature and pressure. At Changi's April normals our calculation puts the effect at about 400 ft, and between 373 and 407 ft across the twelve months. The dew point sets the size of it far more than the air temperature does. The FAA pamphlet says humidity "is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency", and that "if high humidity does exist, however, it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate." Those are the FAA's words; this page gives no performance figures of its own.
It is a training shortcut, not a figure from the FAA pamphlet. In our calculation the standard atmosphere's own factor is 118.6 ft per °C at every altitude when the air is at standard temperature, and it gets smaller as the air warms: 111.5 ft per °C averaged over 30 °C above standard at sea level. So in heat the rule reads high for dry air, and it ignores humidity, which pulls the other way. At Changi's April normals the rule lands 301 ft below the full calculation. The calculator shows the rule's own figure and its gap for whatever you enter.
QNH, in hectopascals or inches of mercury. Singapore's AIP defines QNH as the "altimeter sub-scale setting to obtain elevation when on the ground", and both Singapore's and Malaysia's AIPs give the altimeter setting in hectopascals. QFE, the pressure at the aerodrome itself, is a different value and is not what this calculator expects. Settings from 930 to 1,070 hPa (27.50 to 31.50 inHg) are accepted; a number in the wrong unit is refused with a message rather than converted silently.
None of the sources this page relies on sets a single cut-off, so the calculator does not label results as moderate or high. It tells you how far the density altitude sits above or below the field elevation and how much humidity contributed. Whether that matters for a particular aircraft, load and runway is answered by that aircraft's flight manual.
No. It is an educational calculation of a property of the air from numbers you type in. It is not an aircraft performance figure, and it is not official weather. Planning uses the aircraft's flight manual, current official weather and the pilot's own judgement.
No. The values you enter are processed entirely in your browser. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs on your device only.
Method & sources
How it computes
Station pressure from QNH (hPa, or inHg × 33.8639) along the standard atmosphere; pressure altitude from that pressure. Vapour pressure from the dew point by the NWS formula 6.11 × 10^(7.5·Td ÷ (237.3 + Td)) hPa; virtual temperature Tv = T ÷ (1 − (e ÷ p) × 0.378); density altitude is the standard-atmosphere height with density p ÷ Tv. No dew point: dry air, labelled. Comparison line only: PA + 120 ft × (OAT − ISA), a training rule of thumb.
What this tool implements
- Full method after the US National Weather Service (El Paso WFO) density altitude formula: inputs air temperature, station pressure and dew point; "the virtual temperature must be calculated first"; vapour pressure e = 6.11 × 10^(7.5·Td ÷ (237.3 + Td)) hPa and Tv = T ÷ (1 − (e ÷ p)(1 − 0.622)). The calculator then takes the standard-atmosphere height with that density instead of the NWS's fitted constants (145366, 17.326, 0.235); at Changi's April normals the NWS constants land 15 ft higher.
- Standard-atmosphere constants assumed, not read from the standard: 288.15 K and 1013.25 hPa at sea level, lapse 6.5 K/km, g 9.80665 m/s², R 287.05287 J/(kg·K) — the commonly reproduced ICAO values (ICAO Doc 7488 and ISO 2533 were not opened). The FAA's pamphlet P-8740-2 confirms 15 °C / 59 °F and 29.92 inHg, "1013 hPa in other parts of the world".
- The 120 ft per °C rule (PA + 120 × (OAT − ISA)) is shown as a comparison line only and labelled a training shortcut; it is not stated in FAA P-8740-2. RECATOOLS calculation: the standard atmosphere's dry factor is 118.6 ft per °C at every altitude at standard temperature and averages 116.1, 113.8 and 111.5 ft per °C over +10, +20 and +30 °C at sea level, so in heat the rule reads high for dry air while omitting humidity; at Changi's April normals it reads 301 ft below the full calculation, and 283–310 ft below across the twelve months.
- Altimeter setting in hPa or inHg: the CAAS (Singapore) and CAAM (Malaysia) AIPs, GEN 2.1, publish the altimeter setting in hectopascals, and CAAS GEN 2.2 defines QNH as the "altimeter sub-scale setting to obtain elevation when on the ground". A number in the other unit is refused with a message. Sanity limits of RECATOOLS's choosing, not physical or regulatory values: QNH 930–1,070 hPa / 27.50–31.50 inHg, field elevation −1,500 to 18,000 ft, air temperature −60 to 60 °C, dew point no higher than the air temperature, RH above 0 and at most 100%.
- Humidity worked example from the MSS Changi 1991–2020 normals: April mean daily maximum 32.4 °C; the 24-hour mean 28.2 °C and 82.6% RH give a dew point near 25.0 °C (our Magnus conversion). At standard pressure and Changi's 6.66 m elevation, dry 2,017 ft and full 2,416 ft: humidity adds 399 ft, and 373–407 ft across the twelve months. Monthly means, not co-occurring hourly values. FAA P-8740-2's humidity wording (an engine-power effect; add 10 percent to computed takeoff distance) is quoted as the FAA's, not as advice from the page.
- No performance bands or thresholds: none of the sources sets a density-altitude cut-off, so the earlier warning bands (5,000 / 8,000 ft) and the "7,000–8,000 ft" light-aircraft claim were removed. The page describes the air, not an aircraft.
Sources
- National Weather Service, El Paso WFO. Density Altitude calculator and formula. https://www.weather.gov/epz/wxcalc_densityaltitude (formula: https://www.weather.gov/media/epz/wxcalc/densityAltitude.pdf)
- Federal Aviation Administration. Density Altitude, pamphlet FAA-P-8740-2 (FAA Safety Team, AFS-8, 2008): https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/…
- Civil Aviation Authority of Singapore. Singapore AIP, eAIP effective 3 September 2026: GEN 2.1 (units), GEN 2.2 (QNH, QFE), AD 2 WSSS and WSSL AD 2.2. https://aim-sg.caas.gov.sg/aip/
- Civil Aviation Authority of Malaysia. Malaysia AIP, eAIP 11 August 2026: GEN 2.1 (altimeter setting in hectopascals), AD 2 WMKK, WMKP and WBKK AD 2.2. https://aip.caam.gov.my/aip/eAIP/2026-08-11/html/
- Meteorological Service Singapore. Climate of Singapore — Changi Climate Station normals 1991–2020. https://www.weather.gov.sg/climate-climate-of-singapore/
- Guinn and Barry (2016). International Journal of Aviation, Aeronautics, and Aerospace 3(3). https://doi.org/10.15394/ijaaa.2016.1124 — abstract read; the full paper was not opened.
What can make this go out of date
- none at runtime (elevation, altimeter setting, temperature and dew point or humidity are typed in by the user)
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