What is Density Altitude?
Density altitude is pressure altitude corrected for temperature — the altitude at which the atmosphere has its current air density, and the figure that determines actual aircraft performance.
When density altitude is high, the air is thin: takeoff rolls lengthen, climb rates drop, engine output falls, and service ceilings decrease. Hot, high, and humid conditions all drive density altitude up.
How to Calculate Density Altitude
To calculate density altitude, apply the standard aviation approximation:
DA = Pressure Altitude + 120 × (OAT − ISA Temperature)
Where OAT is outside air temperature in °C, and ISA Temperature is the standard temperature at your pressure altitude (15°C at sea level, decreasing by 2°C per 1,000 ft). The factor 120 ft/°C is the standard density altitude correction coefficient. A positive result means the air is less dense than standard — performance is degraded.
Step-by-Step Example
Enter your field elevation, altimeter setting, and OAT into the density altitude calculator above for an instant result, or work through it manually:
Suppose you are departing a field at 3,000 ft elevation, the altimeter setting is 29.82 inHg, and the OAT is 30°C.
- Pressure Altitude: PA = 3,000 + (29.92 − 29.82) × 1,000 = 3,000 + 100 = 3,100 ft
- ISA Temp at 3,100 ft: 15 − (3.1 × 2) = 15 − 6.2 ≈ 8.8°C
- Density Altitude: DA = 3,100 + 120 × (30 − 8.8) = 3,100 + 120 × 21.2 = 3,100 + 2,544 ≈ 5,644 ft
Even though the field sits at 3,000 ft, your aircraft is performing as if it were at nearly 5,700 ft on a standard day — a significant performance penalty on a warm morning.
When Density Altitude Matters
Density altitude is most critical at airports that are already at high elevation — places like Denver, Aspen, or mountain strips in the Rockies and Alps. On hot summer days, a sea-level airport can briefly reach density altitudes of 2,000–3,000 ft, adding hundreds of feet to your takeoff roll. Pilots must check density altitude before every departure for performance calculations: required runway length, climb gradient over obstacles, maximum takeoff weight, and single-engine service ceiling all degrade as density altitude increases. Many GA accidents involve pilots who underestimated density altitude at high-elevation airports in summer. Use the density altitude calculator at the top of this page to check your value before consulting your aircraft's performance charts.
Frequently Asked Questions
- What is a density altitude calculator?
- A density altitude calculator takes your field elevation, altimeter setting (QNH), and outside air temperature (OAT) as inputs and instantly returns density altitude in feet. It replaces the two-step manual process — first solving for pressure altitude, then applying the temperature correction — reducing the chance of arithmetic errors during preflight planning.
- What is the difference between pressure altitude and density altitude?
- Pressure altitude is your altitude assuming sea-level pressure is exactly 29.92 inHg — it ignores temperature. Density altitude goes one step further by correcting for the actual temperature: hot air is less dense, which raises the density altitude above the pressure altitude. Cold air is denser, which lowers it. Density altitude is what determines aircraft performance; pressure altitude is the intermediate step to calculate it.
- What density altitude is dangerous?
- There is no single threshold, but most piston aircraft experience noticeable performance degradation above 5,000–6,000 ft density altitude. Above 8,000 ft, normally-aspirated engines are significantly down on power. High-altitude airports like Leadville, CO (9,927 ft elevation) can see density altitudes exceeding 12,000 ft on hot days — conditions that require careful weight and performance planning and may require turbocharged aircraft.
- How do I find density altitude without a calculator?
- The E6B flight computer (physical or electronic) has a density altitude window on the circular slide rule side. Set the pressure altitude on the outer ring against the OAT on the inner ring, and read density altitude from the indicated window. As a rough mental math rule, add 120 ft of density altitude for every degree Celsius above the ISA standard temperature at your pressure altitude.