03 · Performance · Density altitude
Density altitude .
The altitude your wing thinks it's flying at — pressure altitude corrected for non-standard temperature. The number that actually governs takeoff roll, climb rate, and service ceiling on a hot day in the mountains.
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Section 01
Compute
Section 02
Why density altitude matters
An aircraft wing flies through air, not altitude. Hot, low-pressure air contains fewer molecules per cubic foot than the standard atmosphere — so a wing moving through it at a given indicated airspeed generates less lift, the engine breathes less oxygen, and the propeller bites less air. Density altitude is the single number that summarises all three effects.
The formula is the FAA Pilot's Handbook of Aeronautical Knowledge approximation:
DA = PA + 120 × (OAT − ISA), where ISA = 15 − 1.98 × PA / 1,000. For
each degree Celsius above the standard-atmosphere value at your pressure altitude, you add 120
ft to the altitude your wing thinks it's flying at. The number can quickly exceed 10,000 ft on
a hot summer afternoon at a mountain field — well above the published service ceiling of some
normally-aspirated singles.
The 120 ft/°C rule is accurate to within ~50 ft up to about 10,000 ft DA — which is the range that matters operationally. Above that, the exponential lapse becomes material; treat the output as a guide rather than a hard number.
Section 03
A worked example
KDEN field elevation on a 30 °C summer afternoon. The pressure altitude is close to the field elevation (about 5,400 ft after a typical altimeter setting). At 30 °C, the air is 25.7 °C hotter than the standard atmosphere expects — your wing performs as if you were at 8,483 ft on a standard day. That's most of the difference between a comfortable departure and the runway-end-of-the-fence kind.
Inputs
- Pressure altitude
- 5,400ft
- Outside air temp
- 30°C
Output
- ISA at 5,400 ft
- 4.3°C
- Deviation from ISA
- +25.7°C
- Density altitude
- 8,483ft
Section 04
Common pitfalls
- 01Field elevation is not pressure altitude. Compute pressure altitude from field elevation and the current altimeter setting first — there is a calculator on this site for that.
- 02OAT means outside air temperature, not cabin temperature and not the temperature reported in a METAR an hour ago. Measure or read it close to the time you intend to use the number.
- 03The 120 ft per Celsius rule starts to diverge above ~10,000 ft DA. If you're computing a service-ceiling case, treat the output as a guide.
- 04Density altitude is a performance number, not a regulatory one. The FARs that reference altitude (cruise altitudes, oxygen requirements) are still keyed to pressure altitude.
- 05Cold density altitudes (below field elevation) help performance but do not change the visual sight picture, the runway length, or the cold-soak gotchas of winter ops.
Download
Free on the App Store. iOS 18 and up.
In the iOS app, density altitude is computed automatically from each watched station's current METAR and field elevation — visible on the station detail screen alongside the other performance numbers.