What is density altitude?

Density altitude is pressure altitude corrected for nonstandard temperature. It is the height in the standard atmosphere where the air would be as thin as the air around you. The higher it is, the worse an aircraft performs: at an airport 500 ft above sea level with a density altitude of 5,000 ft, aircraft perform as if the airport were at 5,000 ft.

What is the difference between pressure altitude and density altitude?

Pressure altitude is the altitude an altimeter shows when it is set to 29.92 inHg (1013 hPa). Density altitude takes pressure altitude and corrects it for a temperature that is warmer or colder than standard. When the temperature is exactly standard for that pressure altitude, the two are the same.

How do I calculate density altitude by hand?

First find the pressure altitude, either by setting an altimeter to 29.92 inHg or by correcting the field elevation for the altimeter setting. Then read the density altitude off an FAA density altitude chart using the pressure altitude and the outside air temperature, or use the National Weather Service formula shown on this page. For a rough figure, the FAA’s Instrument Flying Handbook adds 120 ft to the pressure altitude for every 1 °C the temperature is above standard.

Does humidity change density altitude?

Yes, a little. Moist air is less dense than dry air, so humidity raises density altitude. In the FAA’s own example, at 8,000 ft and 80 °F, a dew point of 75 °F gives a density altitude of 11,564 ft, almost 500 ft higher than with dry air. The FAA says there is no rule of thumb for humidity, so add the dew point to the calculator to include it.

When do weather reports mention density altitude?

ASOS and AWOS phone and radio broadcasts include the density altitude when it is more than 1,000 ft above the field elevation. ATIS broadcasts may add a “check density altitude” advisory when appropriate.

Is density altitude on the Part 107 test?

Yes. The FAA’s Remote Pilot Airman Certification Standards list density altitude under the effects of weather on performance (UA.III.B.K1a), and the FAA’s Remote Pilot study guide covers it in its chapter on weather and small unmanned aircraft performance.

Is the calculator free?

Yes. There is no account to create and nothing to install. The calculation runs in your browser.

Density altitude calculator

Work out density altitude and pressure altitude from the field elevation, altimeter setting and temperature. Add the dew point to include humidity, or paste a METAR to fill in the readings. No signup.

Field elevation

The height of the ground above sea level, from a chart or the airport listing.

Altimeter setting

From the METAR (A3003 means 30.03), ATIS or AWOS.

Temperature

Dew point (optional)

Fill in from a METAR

Paste a METAR. The temperature, dew point and altimeter setting are read from it. A METAR does not give the field elevation, so enter that yourself.

Estimated density altitude

7,735 ft

1,852 ft above the field elevation

Pressure altitude: 5,726 ft
Air density vs standard sea level: 79%
Standard temperature at pressure altitude: 38.6 °F
Difference from standard: +31.4 °F

No dew point, so this assumes dry air. Humid air is less dense, so add the dew point to include it.

More than 1,000 ft above the field elevation. Airport ASOS and AWOS phone and radio broadcasts add the density altitude to their reports when it is this high.

Filled in with the sample problem from Chapter 11 of the FAA’s Pilot’s Handbook of Aeronautical Knowledge, where the FAA’s chart gives about 7,700 ft. Change any value to run your own numbers.

What density altitude is

The FAA defines density altitude as pressure altitude corrected for nonstandard temperature. Put simply, it is the height in the standard atmosphere where the air would be as thin as the air around you right now.

It matters because aircraft performance follows air density. When density altitude goes up, performance goes down. The FAA’s example: if an airport at 500 ft has a reported density altitude of 5,000 ft, aircraft there perform as if the airport were at 5,000 ft.

High

The higher the airport or launch site, the thinner the air. At high airports, such as those in the western United States, heat sometimes pushes density altitude so high that safe operations are impossible.

Hot

Warm air is less dense. When the temperature rises above standard for a place, density altitude goes up, which is why the FAA suggests flying in the cooler hours when performance is in question.

Humid

Water vapor is lighter than dry air, so moist air is less dense. The FAA says humidity is not generally a major factor, but it must be taken into account.

Density altitude and your drone

Drones fly in the same air as airplanes. The FAA’s Remote Pilot study guide covers density altitude in its chapter on weather and small unmanned aircraft performance: thinner air lowers performance, and the higher you fly, the more power the aircraft needs and the less it has available.

It is also on the Part 107 knowledge test. The FAA lists density altitude under the effects of weather on performance in the Remote Pilot Airman Certification Standards. Try a question on it in our Part 107 practice test.

The FAA says a pilot’s first reference for performance information is the manufacturer’s manual for the aircraft. For a drone, that means the limits it lists, such as maximum takeoff altitude and operating temperatures, and any guidance on payload. Read each limit the way the manual defines it. A maximum takeoff altitude, for example, is not necessarily a density altitude.

How to calculate density altitude: the formula

The calculator uses the formulas behind the National Weather Service density altitude calculator, which the FAA’s handbook uses in its humidity example. It works in four steps.

1. Find the station pressure
Turn the altimeter setting into the actual air pressure at the field elevation.
P = altimeter × ((288 − 0.0065 × h) ÷ 288)^5.2561
h is the field elevation in meters. P comes out in the same unit as the altimeter setting.
2. Find the pressure altitude
Look up the height in the standard atmosphere that has this pressure.
PA = (1 − (P ÷ 1013.25)^0.190284) × 145,366.45
P is the station pressure in hectopascals (millibars). The answer is in feet.
3. Allow for humidity (optional)
With a dew point, work out the vapor pressure, then the virtual temperature: the temperature at which dry air at the same pressure would be as dense as the moist air.
e = 6.11 × 10^(7.5 × Td ÷ (237.3 + Td)) Tv = T ÷ (1 − (e ÷ P) × 0.378)
Td is the dew point in °C, T the temperature in kelvin and P the station pressure in hectopascals. Without a dew point, Tv is simply T.
4. Find the density altitude
Put the station pressure and the virtual temperature into the density altitude formula.
DA = 145,366 × (1 − (17.326 × P ÷ Tv)^0.235)
P is the station pressure in inches of mercury and Tv is in degrees Rankine (kelvin × 1.8). The answer is in feet.

Worked example from the FAA

The FAA’s Pilot’s Handbook takes a station pressure of 22.22 inHg (the standard pressure at 8,000 ft), a temperature of 80 °F and a dew point of 75 °F. The density altitude comes out at 11,564 ft. With dry air it would be almost 500 ft lower. These formulas give 11,565 ft, and 11,092 ft for dry air.

A quick estimate without a calculator

Up to 10,000 ft, pressure drops by about 1 inHg for every 1,000 ft. So a rough pressure altitude is the field elevation plus 1,000 ft for every inch the altimeter setting is below 29.92, or minus 1,000 ft for every inch above it.

For density altitude, the FAA’s Instrument Flying Handbook gives a rule of thumb for when no chart is at hand: add 120 ft for every 1 °C the temperature is above the standard temperature at that pressure altitude. Standard temperature is 15 °C (59 °F) at sea level and drops by about 2 °C (3.5 °F) for every 1,000 ft.

The handbook’s example: at a pressure altitude of 3,000 ft, standard temperature is 9 °C. At 20 °C the air is 11 °C warmer than standard, and 11 × 120 ft adds 1,320 ft, for a density altitude of about 4,320 ft. The calculator gives 4,286 ft for the same conditions, so the shortcut is close but not exact.

Pressure altitude

Pressure altitude is the altitude an altimeter shows when it is set to 29.92 inHg (1013 hPa), the standard sea level pressure. It is the basis for aircraft performance charts. In the United States, every aircraft at or above 18,000 ft MSL sets its altimeter to 29.92 inHg, so up there everyone flies by pressure altitude, in flight levels.

The FAA gives three ways to find it: set the altimeter to 29.92 and read it, correct the field elevation for the current altimeter setting, or use a flight computer. The calculator above shows it next to the density altitude.

Density altitude chart

The FAA’s rule-of-thumb chart shows how quickly heat adds up. Values are density altitude in feet for each field elevation and temperature. For your own conditions, use the calculator.

FAA density altitude rule-of-thumb chart, in feet
Elevation	Standard temp	80 °F	90 °F	100 °F	110 °F	120 °F	130 °F
Sea level	59 °F	1,200	1,900	2,500	3,200	3,800	4,400
2,000 ft	52 °F	3,800	4,400	5,000	5,600	6,200	6,800
4,000 ft	45 °F	6,300	6,900	7,500	8,100	8,700	9,400
6,000 ft	38 °F	8,600	9,200	9,800	10,400	11,000	11,600
8,000 ft	31 °F	11,100	11,700	12,300	12,800	13,300	13,800

Source: FAA, Density Altitude (FAA-P-8740-2), Density Altitude Rule-of-Thumb Chart.

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