Density Altitude Calculator
Why density altitude matters
Density altitude (DA) is a way to express air density as an “equivalent altitude.” When the air is less dense—because you’re at a higher elevation, the temperature is warmer than standard, or pressure is lower—your aircraft performs as if it were operating at a higher altitude than the runway elevation. In practical terms, higher density altitude generally means:
- Longer takeoff roll and reduced acceleration
- Lower climb rate and degraded obstacle clearance
- Reduced propeller/rotor efficiency
- Reduced engine power for normally aspirated engines
This calculator provides an estimate suitable for planning and cross-checking. For actual go/no-go decisions, always use your aircraft’s POH/AFM performance charts and current official weather.
What to enter (and where to get it)
- Field elevation (MSL): Airport elevation above mean sea level (not AGL). Use the chart supplement/airport diagram or the airport info block.
- Outside air temperature (OAT): Temperature at the field (typically from METAR). Ensure the unit matches your selection (°C or °F).
- Altimeter setting: The altimeter setting from METAR/ATIS (e.g., 29.92 inHg or 1013 hPa). This is not the same as “station pressure.”
How the calculator estimates density altitude
A common training and preflight method is to compute pressure altitude first, then apply a temperature correction relative to the International Standard Atmosphere (ISA).
Step 1: Pressure altitude (PA)
Pressure altitude is the altitude indicated when the altimeter is set to standard pressure (29.92 inHg / 1013.25 hPa). A widely used approximation is:
where PA and Elev are in feet and Alt is the altimeter setting in inHg. If you enter meters or hPa, the calculator converts to these working units internally, then converts the final result back to feet and meters for display.
Step 2: ISA temperature at that altitude
ISA defines a standard sea-level temperature of 15 °C and a typical lapse rate near sea level of about 2 °C per 1,000 ft. A simple standard-temperature estimate at the pressure altitude is:
ISA Temp (°C) ≈ 15 − 2 × (PA/1000)
Step 3: Density altitude (DA) via temperature correction
The common rule-of-thumb correction is about 120 ft per °C of temperature difference from ISA:
DA ≈ PA + 120 × (OAT − ISA Temp)
Interpreting the result
- DA close to field elevation: Conditions are near standard; performance should be closer to book values for that elevation.
- DA much higher than field elevation: Expect materially worse takeoff/climb performance. Use POH/AFM charts and consider runway length, obstacles, weight, and density altitude limits.
- Negative DA: Can occur in very cold/high-pressure conditions; performance may improve versus standard, but still confirm with charts and operational considerations.
Worked example
Given:
- Field elevation: 5,000 ft MSL
- Altimeter setting: 29.62 inHg
- OAT: 30 °C
1) Pressure altitude:
PA = 5,000 + (29.92 − 29.62) × 1000 = 5,000 + 0.30 × 1000 = 5,300 ft
2) ISA temperature at PA:
ISA Temp ≈ 15 − 2 × (5.3) = 15 − 10.6 = 4.4 °C
3) Density altitude:
DA ≈ 5,300 + 120 × (30 − 4.4) = 5,300 + 120 × 25.6 = 5,300 + 3,072 = 8,372 ft (≈ 2,552 m)
This means the aircraft will perform roughly like it’s taking off at about 8,400 ft under ISA conditions, even though the runway sits at 5,000 ft.
Quick comparison: how conditions change DA
| Condition change | What happens to air density? | Effect on density altitude |
|---|---|---|
| Higher field elevation | Lower pressure and density | DA increases |
| Warmer-than-ISA temperature | Molecules spread out; density drops | DA increases (often a lot) |
| Lower altimeter setting (lower pressure) | Less pressure compressing the air | DA increases |
| Colder-than-ISA temperature | Air is denser | DA decreases |
| Higher altimeter setting (higher pressure) | Air is more compressed | DA decreases |
Limitations, assumptions, and safety notes
- Approximation method: Uses common training “rule-of-thumb” relationships (ISA lapse approximation and ~120 ft/°C correction). Real-atmosphere behavior and aircraft performance charts may differ.
- Humidity not included: High humidity reduces density and can increase DA slightly; this calculator ignores humidity, so it can understate DA in hot/humid conditions.
- Altimeter setting vs station pressure: METAR altimeter setting is not the same as station pressure; using the wrong value can shift results.
- Valid ranges: Extreme temperatures and very high elevations can reduce the accuracy of simple lapse-rate assumptions.
- Rounding & conversions: Unit conversions and rounding can create small differences versus avionics/EFB tools that use more detailed models.
- Not operational authorization: This tool is for estimation and education. Always confirm takeoff/climb performance with POH/AFM data, runway slope/condition, wind, obstacles, aircraft configuration, and regulatory/operator procedures.