Density Altitude Calculator: Pressure Altitude and ISA Deviation
Enter the aerodrome elevation, the QNH and the temperature, and the calculator gives the pressure altitude, the ISA deviation and the density altitude: the height at which your aircraft will perform as if it were flying in the standard atmosphere. Add the dew point and it counts the humidity too.
Density altitude calculator
Enter the elevation, QNH and temperature to see the result.
Method: the ICAO Standard Atmosphere (Manual of the ICAO Standard Atmosphere, Doc 7488): 1013.25 hPa and 15 °C at sea level, falling 1.98 °C per 1,000 ft. The station pressure is QNH reduced to the aerodrome elevation through that atmosphere; the pressure altitude is the standard height of that pressure; the air density follows from the gas law, with the dew point's water vapour when given; the density altitude is the standard height of that density, as in the US National Weather Service's formula. Effects and the rule of thumb: FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C), FAA pamphlet P-8740-2 Density Altitude, and AOPA.
Pressure altitude and density altitude
The International Standard Atmosphere (ISA) is a model: at sea level a pressure of 1013.25 hPa (29.92 inHg) and a temperature of 15 °C, with the temperature falling by 1.98 °C per 1,000 ft up to 36,090 ft. Real days are never exactly standard, and two numbers measure how far they are.
Pressure altitude is the height in the standard atmosphere at which the pressure is what it is at your aerodrome. It is what your altimeter reads when set to 1013 hPa or 29.92 inHg. A low QNH puts the pressure altitude above the elevation; a high QNH, below it.
Density altitude is pressure altitude corrected for the temperature: the height in the standard atmosphere at which the air is as dense as it is at your aerodrome. Air that is warmer than standard is thinner, so the density altitude rises above the pressure altitude, and the aircraft behaves as if it were at that higher aerodrome.
| Pressure altitude | ISA temperature | ISA pressure |
|---|---|---|
| 0 ft | 15.0 °C | 1013 hPa |
| 2,000 ft | 11.0 °C | 942 hPa |
| 5,000 ft | 5.1 °C | 843 hPa |
| 8,000 ft | −0.8 °C | 753 hPa |
| 10,000 ft | −4.8 °C | 697 hPa |
The formulas and the rules of thumb
- Pressure altitude = elevation + (1013 − QNH) × 30 ft, or in inches, elevation + (29.92 − altimeter setting) × 1,000 ft. One hectopascal is worth about 27 ft near sea level and more higher up; many exams use 27 or 30 ft.
- ISA temperature at that pressure altitude = 15 − 2 × (pressure altitude in thousands of feet) °C.
- ISA deviation = the outside air temperature − the ISA temperature.
- Density altitude ≈ pressure altitude + 120 ft for every degree of ISA deviation.
Worked example: an aerodrome at 5,000 ft, QNH 1013, 30 °C. The pressure altitude is 5,000 ft, where ISA is 5 °C, so the air is ISA +25. The rule of thumb gives 5,000 + 25 × 120 = 8,000 ft; the exact calculation gives about 7,800 ft. The rule reads a little high on hot days, which errs on the safe side. The calculator shows both.
What a high density altitude does
- Less engine power. A normally aspirated piston engine breathes less air by mass and makes less power. A turbocharged engine keeps its power higher, but the wing and the propeller still feel the thin air.
- Less thrust. A propeller moves less air by mass. A jet engine’s take-off thrust falls with pressure altitude and, once the temperature passes its flat-rating limit, with every further degree.
- Higher true airspeed for the same indicated airspeed. The wing needs the same indicated airspeed to fly, so the aircraft reaches it at a higher true airspeed and groundspeed: a longer take-off run and a longer landing roll.
- A lower rate and angle of climb, which matters most with obstacles after take-off.
High, hot and heavy is the classic trap: a high aerodrome on a hot afternoon with a full aircraft. Take off early in the day when you can, check the performance charts for the actual pressure altitude and temperature, and know the distance at which you will abort the take-off. The Library explains the factors affecting take-off and landing performance and climb performance.
Humidity
A molecule of water vapour is lighter than the nitrogen and oxygen it replaces, so humid air is less dense than dry air at the same pressure and temperature. The effect is small in cool air, but on a hot, humid day it can add a few hundred feet to the density altitude, and it also costs a piston engine some power. Most light-aircraft performance charts leave humidity out, so leave a margin on muggy days. Enter the dew point above to see the difference it makes.
Frequently asked questions
What is density altitude? Pressure altitude corrected for non-standard temperature: the height in the International Standard Atmosphere at which the air is as dense as it is where you are. When the air is warmer than standard, the density altitude is above the pressure altitude, and the aircraft performs as if it were at that higher altitude.
How do you calculate density altitude? Work out the pressure altitude (elevation + (1013 − QNH) × 30 ft, or elevation + (29.92 − altimeter setting) × 1,000 ft), find the ISA temperature there (15 °C minus 2 °C per 1,000 ft), and add about 120 ft for every degree the outside air is warmer than that. The calculator on this page uses the exact standard-atmosphere formulas and shows the rule of thumb next to them.
What is the difference between pressure altitude and density altitude? Pressure altitude corrects the elevation for a pressure that is not standard: it is what the altimeter reads set to 1013 hPa or 29.92 inHg. Density altitude also corrects for a temperature that is not standard. They are the same only when the temperature is exactly ISA.
Why does density altitude matter to pilots? Thin air reduces engine power, propeller or jet thrust and the lift at a given true airspeed. The aircraft needs a longer take-off run, lands faster over the ground and climbs more slowly, which matters most at high aerodromes on hot days with a heavy aircraft.
Does humidity affect density altitude? Yes. Water vapour is lighter than dry air, so humid air is less dense. The effect is small in cool air but can reach a few hundred feet on a hot, humid day. Most light-aircraft performance charts do not include it; enter the dew point in the calculator to see it.
Know it before the examiner asks
Wind components, the standard atmosphere and performance come up in the ATPL theory exams, the FAA written tests and airline interviews. v1prep has 16,000+ questions with every answer cited to its source.
Start practising free →