What density altitude actually is
Density altitude is pressure altitude corrected for non-standard temperature. It's the altitude your airplane thinks it's flying at — the altitude in the standard atmosphere that has the same air density as the air you're actually sitting in. When the air gets thinner (hot, high, humid), your wings produce less lift, your prop bites less air, and your normally-aspirated engine makes less power. The number that captures all of this in one figure is density altitude.
This matters because aircraft performance charts in your POH are built around it. Takeoff distance, climb rate, service ceiling, and true airspeed all key off density altitude. FAR 91.103 makes preflight performance computation a legal requirement for every flight — and density altitude is the input.
The two-step calculation
The FAA expects you to compute density altitude in two steps: pressure altitude first, then correct for temperature deviation from standard.
Step 1 — Pressure altitude
Pressure Altitude (PA) = Field Elevation + (29.92 − Altimeter Setting) × 1,000
If the altimeter setting is below 29.92, pressure altitude is higher than field elevation (low pressure = thin air). If it's above 29.92, pressure altitude is lower. The quick alternative on the ramp: set 29.92 in your Kollsman window and read pressure altitude directly off the altimeter.
Step 2 — Correct for temperature
Density Altitude (DA) ≈ PA + 120 × (OAT − ISA Temp)
Where:
- OAT = outside air temperature in °C
- ISA Temp = standard temperature at that pressure altitude
- ISA at sea level = 15°C, decreasing 2°C per 1,000 ft
The 120 ft per °C factor is the working approximation pilots use; an E6B or electronic calculator will give you a slightly more precise answer, but the difference is rarely operationally meaningful.
Worked example — a hot day at a mountain airport
Let's compute density altitude for a Cessna 172 sitting on the ramp at Leadville, Colorado (KLXV) on a summer afternoon.
- Field elevation: 9,934 ft
- Altimeter setting: 30.12 in Hg
- OAT: 25°C
Pressure altitude:
PA = 9,934 + (29.92 − 30.12) × 1,000
PA = 9,934 + (−200)
PA = 9,734 ft
ISA temperature at 9,734 ft: