What is density altitude?

Density altitude is pressure altitude corrected for nonstandard temperature. It is the altitude in the standard atmosphere that has the same air density as the air you are in, and it is the altitude your wings, propeller and engine actually perform at.

How do I calculate density altitude quickly?

Find pressure altitude: field elevation plus 1,000 feet for every inch the altimeter is below 29.92. Find standard temperature: 15 degrees C minus 2 degrees per 1,000 feet. Then add about 120 feet of density altitude for every degree C the outside air is warmer than standard.

Does humidity affect density altitude?

Yes. Moist air is less dense than dry air, so humidity raises density altitude a little further. This formula, like most flight computers, assumes dry air. On hot, humid days expect slightly worse performance than the number shown.

What density altitude is too high?

There is no single number. Look up takeoff distance and climb rate in your POH for the density altitude you get, at your weight, and compare them with the runway and the terrain past it. Many pilots set a personal density altitude limit for each airplane they fly.

Pilot tool

Density altitude calculator

Density altitude is pressure altitude corrected for temperature. Start from field elevation, add 1,000 feet for every inch of mercury the altimeter is below 29.92, then add about 120 feet for every degree Celsius the air is warmer than standard. The calculator below does it for you.

Density altitude

8,090ft

3,090 ft above field elevation

Pressure altitude

5,100ft

ISA temperature

4.8°C

ISA deviation

+25.2°C

Warmer than standard

OAT

30.0°C

How it works

Your airplane does not care how high the airport is. It cares how thick the air is. Thin air means less lift for a given airspeed, less thrust from the propeller and less power from a normally aspirated engine, so takeoff rolls get longer and climb rates get shallower. Density altitude is a single number for that: the altitude in the standard atmosphere where the air would be this thin.

Two things move it. Pressure: a low altimeter setting means less air above you, which is why pressure altitude is the starting point. Temperature: warm air expands, so a hot afternoon can put a 5,000 foot airport well above 8,000 feet in performance terms. Humidity pushes it a little higher again, and this formula ignores that, so treat the result as slightly optimistic on humid days.

Take the result to the performance charts in your POH. That is where density altitude becomes a takeoff distance, a climb rate and a yes or no for the runway in front of you.

The formula

Check the math yourself.

Pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000

ISA temperature = 15 − 2 × (pressure altitude ÷ 1,000) °C

Density altitude ≈ pressure altitude + 118.8 × (OAT − ISA temperature)

The definitions, the 1 inch per 1,000 feet pressure lapse and the 2 °C per 1,000 feet temperature lapse are from the sources below. The 118.8 feet per degree factor is a widely used linear approximation of the standard atmosphere, in place of reading the density altitude chart (PHAK Figure 11-4). It assumes dry air; cross-check with the chart or your flight computer when the margin is thin.

Source

Worked example

A summer afternoon at an airport 5,000 feet above sea level. The METAR reports 30 °C and altimeter 29.82.

Pressure altitude = 5,000 + (29.92 − 29.82) × 1,000 = 5,100 feet. Standard temperature at 5,100 feet = 15 − 2 × 5.1 = 4.8 °C. The air is 25.2 °C warmer than standard.

Density altitude = 5,100 + 118.8 × 25.2 ≈ 8,090 feet. The airplane will perform as if it were taking off from an airport about 3,100 feet higher than the one it is on. Look up the takeoff distance at 8,000 feet in the POH, not at 5,000.

Questions

What is density altitude?: Density altitude is pressure altitude corrected for nonstandard temperature. It is the altitude in the standard atmosphere that has the same air density as the air you are in, and it is the altitude your wings, propeller and engine actually perform at.
How do I calculate density altitude quickly?: Find pressure altitude: field elevation plus 1,000 feet for every inch the altimeter is below 29.92. Find standard temperature: 15 degrees C minus 2 degrees per 1,000 feet. Then add about 120 feet of density altitude for every degree C the outside air is warmer than standard.
Does humidity affect density altitude?: Yes. Moist air is less dense than dry air, so humidity raises density altitude a little further. This formula, like most flight computers, assumes dry air. On hot, humid days expect slightly worse performance than the number shown.
What density altitude is too high?: There is no single number. Look up takeoff distance and climb rate in your POH for the density altitude you get, at your weight, and compare them with the runway and the terrain past it. Many pilots set a personal density altitude limit for each airplane they fly.

More pilot tools

From density altitude to runway margin.

In Ascent 1.9.6, Copilot (beta) checks density altitude against your personal minimums and reads takeoff distance from your own POH tables for the current METAR. It is advisory; the decision stays with you.