Density altitude calculator
Enter field elevation, QNH and outside air temperature – the calculator shows pressure altitude, the deviation from ISA and the density altitude your aircraft actually “feels”. Optional with dew point for humid air.
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What density altitude means
Density altitude is the altitude in the standard atmosphere (ISA) at which the air has the same density as the air around your aircraft right now. Wings, propeller and a normally aspirated engine all depend on air density, so on a hot day an airfield at 5,600 ft can perform like one at 8,000 ft.
Two steps lead there. First the pressure altitude: the altitude in the ISA at which the measured pressure occurs. It is field elevation corrected for the QNH – roughly 27–30 ft per hPa below 1013.25 hPa. Then the temperature: every degree above the ISA temperature at that pressure altitude makes the air thinner.
Formulas used
Rule of thumb: pressure altitude ≈ elevation + (1013 − QNH) × 30 ft; ISA temperature ≈ 15 °C − 2 °C per 1,000 ft; density altitude ≈ pressure altitude + 120 ft × (OAT − ISA temperature).
The calculator uses the exact ISA relations: station pressure from QNH and elevation, air density from pressure and (virtual) temperature, and the ISA altitude with that density. The rule-of-thumb result is shown for comparison – it is usually 100–200 ft higher than the exact value in summer.
Example: Samedan on a summer day
Samedan (LSZS) lies at about 5,600 ft. With QNH 1013 hPa and 25 °C the pressure altitude is 5,607 ft and the ISA temperature there is 3.9 °C – the air is 21 °C warmer than standard. Density altitude: about 8,000 ft (exact) or 8,140 ft with the rule of thumb. The air density is 79 % of sea level, and a normally aspirated engine delivers only about three quarters of its rated power.
With a dew point of 12 °C the density altitude rises by another 200 ft. Humidity matters less than temperature, but on muggy days it adds a few hundred feet.
What it does to performance
Higher density altitude means less engine power, less propeller thrust and a higher true airspeed for the same indicated airspeed. Takeoff roll and landing distance grow, the rate of climb drops. The UK CAA's Safety Sense Leaflet 7 gives rough factors of +10 % takeoff distance per 1,000 ft of aerodrome elevation and +10 % per 10 °C above ISA – but only the POH charts give the real numbers for your aircraft.
BriefingPal reads those charts for more than 100 aircraft: enter departure, temperature and QNH, and the app computes takeoff and landing distance for your loaded aircraft and compares them with the runway length.
Frequently asked questions
What is the difference between pressure altitude and density altitude?
Pressure altitude only corrects the elevation for non-standard pressure (QNH). Density altitude additionally corrects for temperature (and humidity). POH performance charts are usually entered with pressure altitude and temperature; density altitude summarises the effect in one number.
How do I calculate density altitude quickly?
Pressure altitude = elevation + (1013 − QNH) × 30 ft. ISA temperature = 15 °C − 2 °C per 1,000 ft of pressure altitude. Density altitude ≈ pressure altitude + 120 ft × (OAT − ISA temperature).
Does humidity increase density altitude?
Yes. Water vapour is lighter than dry air, so humid air is less dense. The effect is a few hundred feet on warm, humid days – smaller than that of temperature, but not negligible.
Why is my density altitude lower than the field elevation?
On cold days with high pressure the air is denser than standard. Density altitude can then be well below elevation, and the aircraft performs better than the book values for ISA.
How much power does a piston engine lose?
A normally aspirated piston engine loses roughly 3 % of its power per 1,000 ft of density altitude. The calculator estimates the remaining power with the Gagg-Ferrar relation (1.132 σ − 0.132).
Which value should I use for takeoff performance?
Use the method of your POH: most charts take pressure altitude and outside air temperature. Density altitude is a good indicator of how demanding the conditions are.
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