How to use the density altitude calculator
- Paste a METAR, or type the weather. A METAR gives the temperature, dew point and altimeter setting (for example 18/M02 and A3001). The calculator fills them in.
- Enter the airport's elevation. A METAR doesn't include it; take it from the airport chart or your sim's airport information. Feet or meters.
- Check the altimeter setting. Inches of mercury in the US (A3001 = 30.01), hectopascals in most other countries (Q1013 = 1013).
- Add the dew point if you have it. Humidity raises the density altitude a little; leave it empty to work it out for dry air.
- Read the answer. The density altitude comes first, then how far above (or below) the airport it is, the pressure altitude, the standard temperature at that height and how dense the air is compared with a standard day at sea level.
What density altitude means
Density altitude is the height in the standard atmosphere where the air is as thin as it is where you are. The FAA's Pilot's Handbook of Aeronautical Knowledge puts it simply: whatever the aircraft's real altitude, it performs as though it were at the density altitude.
Thin air takes away three things at once:
- Power, because the engine takes in less air.
- Thrust, because a propeller is less efficient in thin air.
- Lift, because thin air exerts less force on the wings.
The handbook lists what makes the density altitude high: high elevation, low pressure, high temperature and high humidity. Some examples from this calculator, for dry air unless a dew point is given:
| Airport | Altimeter | Temperature | Density altitude | Air density |
|---|---|---|---|---|
| Sea level | 29.92 | 15 °C (standard) | about 0 ft | 100% |
| Sea level | 29.92 | 35 °C | 2,292 ft | 94% |
| Sea level | 29.92 | 35 °C, dew point 25 °C | 2,692 ft | 92% |
| 2,000 ft | 30.00 | 30 °C | 4,089 ft | 89% |
| 5,000 ft | 30.00 | 15 °C | 6,081 ft | 83% |
| 5,000 ft | 30.00 | 32 °C | 7,944 ft | 79% |
| 8,000 ft | 30.10 | 25 °C | 10,726 ft | 72% |
| 1,000 ft | 30.60 | −20 °C | −3,988 ft | 112% |
On a hot day the same airport behaves like one thousands of feet higher. On a cold, high-pressure day the density altitude can drop below sea level, and the aircraft performs better than its sea-level figures.
Pressure altitude and the standard atmosphere
Aircraft performance figures are written for the standard atmosphere: 29.92 inches of mercury (1013.2 hPa) and 15 °C at sea level, with the temperature falling about 2 °C for every 1,000 feet you climb.
Pressure altitude is the height above the level where the pressure is 29.92. Set 29.92 in the altimeter and it reads the pressure altitude. When the day's pressure is higher than standard, the pressure altitude is below the airport's elevation; when it is lower, above. The calculator works it out from the elevation and the altimeter setting.
Density altitude is the pressure altitude corrected for a temperature that isn't standard. The calculator shows the standard temperature at the pressure altitude and how far today is above or below it: the further above, the higher the density altitude.
Performance charts in an aircraft's manual usually start from the pressure altitude and the outside air temperature, so both numbers here are the ones to take to the charts.
Does humidity matter?
A little. Water vapor is lighter than dry air, so moist air is less dense and the density altitude rises. The FAA handbook calls humidity a contributing factor rather than an important one, and gives an example: at a station pressure of 22.22 inches, 80 °F and a dew point of 75 °F, the density altitude is 11,564 feet; with no humidity it would be almost 500 feet lower. This calculator gives 11,565 feet for the same inputs.
Humidity matters most on hot, muggy days, because warm air can hold much more water. On a cold day it is around 100 feet or less.
How it's worked out
The calculator uses the formulas the US National Weather Service publishes for its own weather calculators:
- Station pressure from the altimeter setting and the airport's elevation: the real air pressure at the airport.
- Pressure altitude from the station pressure.
- Vapor pressure from the dew point: how much of the pressure is water vapor.
- Virtual temperature: the temperature dry air would need to be as light as the moist air.
- Density altitude from the station pressure and the virtual temperature.
Checked against the FAA handbook: its humidity example above comes out within 1 foot, and its chart example (5,883 feet, 30.10 inches) gives a pressure altitude of 5,726 feet against the 5,718 its table rounds to. The calculator opens on that chart example, at 70 °F.
Flying high and hot in a sim
Density altitude is easy to try in a sim. Pick a high airport, set a hot day in the weather settings, and fly the same takeoff you'd fly at sea level on a standard day: watch how much longer the takeoff roll is and how slowly the aircraft climbs.
For a realistic flight, take the pressure altitude and the temperature from this calculator to the aircraft's performance charts, if your add-on includes them, and work out the takeoff distance before you line up. Fly live weather and paste the airport's METAR here first, so you know what the air will give you before you start the takeoff roll.