Performance

Density Altitude: What It Is and How to Calculate It

✓ Reviewed against current FAA sources on October 5, 2026

Density altitude is the single most useful performance number in aviation and the one most often misunderstood. It explains why an airplane that leaps off a cold sea-level runway in January struggles on the same runway in July, and it is examined on every pilot knowledge test. Here is what it is, how to work it out with the density altitude calculator or by hand, and what it actually does to the aircraft.

Start with pressure altitude

Pressure altitude is the altitude indicated when the altimeter is set to 29.92 inches of mercury. You can set 29.92 and read it, or compute it:

Pressure altitude = field elevation + (29.92 minus the altimeter setting) x 1,000

The sign takes care of itself. A setting lower than 29.92 means lower than standard pressure and therefore a higher pressure altitude. At a 2,500-foot field with an altimeter setting of 29.42, pressure altitude is 2,500 + 500 = 3,000 feet.

Then correct for temperature

Density altitude is pressure altitude corrected for non-standard temperature. It is the altitude in the standard atmosphere at which the air has the density the aircraft is actually experiencing, which is to say the altitude the airplane thinks it is at.

The working approximation:

Density altitude = pressure altitude + [120 x (OAT minus ISA temperature)]

where ISA temperature at a given pressure altitude is 15 minus (2 x pressure altitude in thousands) degrees Celsius.

Continue the example. At a pressure altitude of 3,000 feet, ISA temperature is 15 minus 6, which is 9 degrees Celsius. If the actual temperature is 30 degrees:

3,000 + [120 x (30 minus 9)] = 3,000 + 2,520 = 5,520 feet

The airplane on that 2,500-foot field will perform as though it were at 5,520 feet. That is the whole point of the number.

The three factors that raise it

• High altitude. Less pressure, less dense air.
• High temperature. Molecules further apart, less dense air.
• High humidity. Water vapor is lighter than dry air, so humid air is less dense at the same pressure and temperature.

That third one is reliably answered backward. Intuition says damp air feels heavy. Physically, a water molecule has less mass than the nitrogen or oxygen molecule it displaces, so moist air is less dense and high humidity therefore increases density altitude and degrades performance. The effect is smaller than the other two but it is real, and it is largest on hot days when the air holds the most moisture.

All three often arrive together, which is why a summer afternoon at a mountain airport is the classic accident setup.

What it does to the airplane

High density altitude degrades performance through three independent mechanisms, and the losses compound:

• The engine makes less power. A normally aspirated engine draws a volume of air. Fewer oxygen molecules in that volume means less fuel burned and less power.
• The propeller makes less thrust. A propeller is a rotating wing and produces less force in less dense air.
• The wing makes less lift at a given true airspeed. Lift varies directly with density, so the airplane must reach a higher true airspeed before the wing will support it.

The consequences you will see:

• takeoff roll is longer, because more true airspeed is needed and less thrust is available to reach it;
• rate and angle of climb are reduced, because there is less excess power;
• true airspeed at liftoff and touchdown is higher, so the airplane rolls further;
• landing distance is longer, for the same reason;
• service ceiling is lower.

The thing that does not change

Indicated airspeed is unaffected. The airspeed indicator measures dynamic pressure, which depends on density in exactly the way lift does, and the two errors cancel.

An airplane stalls at the same indicated airspeed at any density altitude, and you fly the same indicated approach and climb speeds. What changes is the true airspeed those indications correspond to, and therefore the ground distance covered. This is why the airplane touches down at the familiar indicated speed on a hot day and still uses far more runway.

Sources: FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge; 14 CFR 91.103 (opens in a new tab).