Density altitude calculator: 3 METAR numbers and your field elevation

Enter the field elevation from the chart, then the altimeter setting, temperature and dew point from the METAR. The calculator runs the US National Weather Service's formulas and shows the density altitude, the pressure altitude and every step in between.

How the number is worked out

Density altitude is the height in the standard atmosphere where the air has the same density as the air at your field. In thinner air the wing and the propeller make less lift and a piston engine makes less power, so the aircraft performs as if it were at that height.

The calculator takes the National Weather Service's three steps in order. First it turns the altimeter setting into station pressure, the pressure at the field itself: altimeter setting × ((288 − 0.0065 × elevation in metres) ÷ 288)^5.2561. Then it finds pressure altitude from station pressure: (1 − (station pressure in hPa ÷ 1013.25)^0.190284) × 145,366.45 ft.

Last it corrects for temperature and moisture. Water vapour weighs less than the dry air it replaces, so the formula swaps the air temperature for a virtual temperature worked out from the dew point. Density altitude is then 145,366 × (1 − (17.326 × station pressure in inHg ÷ virtual temperature in degrees Rankine)^0.235) ft.

Take a summer afternoon at a field 5,434 ft up, Denver International's elevation, with 30.00 inHg, 32 °C and a dew point of 4 °C. The pressure altitude comes out at 5,365 ft and the density altitude at about 8,590 ft, 3,150 ft above the field. The air there is 77 percent as dense as at standard sea level.

Humidity counts for more than most pilots expect on a hot day. At sea level with 29.92 inHg and 40 °C, dry air gives a density altitude of about 2,830 ft, and a dew point of 25 °C lifts it to about 3,230 ft.

The ground-school shortcut adds 120 ft to the pressure altitude for every degree above the standard temperature. For the Denver afternoon it gives 8,680 ft, about 90 ft above the formula, and it leaves humidity out.

What the number leaves out

• Your takeoff and landing distances. Light aircraft flight manuals usually tabulate performance against pressure altitude and outside air temperature, so take the pressure altitude from here to your flight manual's charts.
• Runway slope, wind, surface, weight and the condition of the aircraft. Each one changes performance, and none of them is in this number.
• The air over the runway. A METAR temperature comes from a shaded screen 1.25 to 2 metres above the ground, and the air over a sunlit runway can run warmer.
• Fine precision. The calculator shows density altitude to the nearest 10 ft. On a standard sea-level day the formula gives about 20 ft rather than zero, which shows the scale it works to.

The formulas are the US National Weather Service's, from its density altitude, station pressure and pressure altitude calculators, with their constants unchanged. A script runs the NWS pages' own code against this calculator on 2,527 combinations of elevation, pressure, temperature and dew point, and they agree to within a hundredth of a foot.

Common questions

What is the difference between pressure altitude and density altitude?

Pressure altitude is the height in the standard atmosphere with the same pressure as your field: what the altimeter reads with 29.92 inHg or 1013.25 hPa set. Density altitude starts from pressure altitude and corrects it for temperature and humidity, so it matches the density of the air.

Does humidity raise density altitude?

Yes. Water vapour is lighter than the nitrogen and oxygen it displaces, so moist air is less dense than dry air at the same pressure and temperature. At sea level on a 40 °C day, a dew point of 25 °C adds about 400 ft to the density altitude.

How do I read the altimeter setting and temperature in a METAR?

In a US METAR the altimeter setting follows an A: A3002 means 30.02 inHg. Elsewhere it follows a Q: Q1013 means 1013 hPa. Temperature and dew point sit together in degrees Celsius, as in 32/04, and an M means minus: 05/M02 is 5 °C with a dew point of minus 2 °C.

Why does the calculator ask for the field elevation?

The altimeter setting is the field's pressure reduced to sea level. The calculator needs the elevation to work back to the pressure at the field itself, the step the National Weather Service calls station pressure.