What is the rule of thumb for density altitude?
Start with pressure altitude: field elevation plus (29.92 minus the altimeter setting) times 1,000. Find the standard temperature there, 15 °C minus 2 °C per 1,000 feet. Then add 120 feet for every degree Celsius the air is warmer than standard, or subtract it when colder.
- Density altitude (rule of thumb)
- 5950 ft
- Pressure altitude
- 4120 ft
- Standard temperature there
- 6.76 °C
E6B: Training aid only, not for navigation or flight. Verify with your POH/AFM and approved sources.
Play
Warm the day up one degree at a time and count 120 feet each step.
Stuck? Pick one of the examples from the “Try an example” menu, or press “New example.”
Understand
Density altitude is a straight-line linear correction, twice:
- Pressure: each inch of mercury below 29.92 adds 1,000 ft.
- Temperature: each degree Celsius above standard adds 120 ft.
The real atmosphere isn't linear. Density falls with a power of altitude. But over the few thousand feet that matter for takeoff, a straight line fits well enough to do in your head.
Use
Every input has a unit menu, so you can type values in the units you already have. Results follow your units.
Show the work
- Pressure altitude: 1,000 ft per inch of mercury
\text{PA} = 4000 + (29.92 - 29.8) \times 1000 = 4120\ \text{ft} - Standard temperature: 15 °C, minus 2 °C per 1,000 ft
T_{\text{ISA}} = 15 - 2 \times \frac{4120}{1000} = 6.76\ ^\circ\mathrm{C} - Add 120 ft per °C above standard
\text{DA} \approx 4120 + 120 \times (22 - 6.76) = 5949\ \text{ft}
Export
Enter field elevation, the altimeter setting, and the temperature. For the exact answer, including humidity, open the exact density altitude tool linked above.
- Use POH performance charts with pressure altitude and temperature for real takeoff and climb numbers.
For learning and estimation. Verify with applicable codes, standards, and a qualified professional before using in design, construction, or safety-critical work.
Cheat card
| Symbol | Meaning | Unit |
|---|---|---|
| pressure altitude | ft | |
| outside air temperature | °C | |
| standard temperature at that altitude | °C |
- Each 0.01 inHg below 29.92 adds about 10 ft of pressure altitude.
- The rule runs a little high on hot days compared with the exact formula, which errs on the safe side.
- Humidity isn't in the rule. On humid days, the exact tool with dew point gives a higher answer.
Where it’s used
- Aerospace
Pilots use the 120-feet-per-degree rule for a quick density altitude check before takeoff. - Driving & Travel
It explains why summer afternoons at mountain airports feel like flying thousands of feet higher.
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Questions people ask
What is the density altitude rule of thumb?
Density altitude ≈ pressure altitude + 120 × (OAT − standard temperature). Standard temperature is 15 °C at sea level, dropping 2 °C per 1,000 ft.
How accurate is the rule of thumb?
Within a few hundred feet for typical conditions. On a 30 °C day in Denver it gives about 8,290 ft, versus about 8,080 ft from the exact dry-air formula. That's a slightly conservative result.
Where does 120 feet per degree come from?
Near the ground, a 1 °C rise in temperature thins the air about as much as climbing roughly 120 ft in the standard atmosphere. It's a straight-line fit to the exact curve.