Density Altitude Calculator

Frequently Asked Questions

How is density altitude calculated?

DA = pressure altitude + 120 × (OAT − ISA temp at that PA). ISA temp at sea level is 15 °C, dropping 2 °C per 1,000 ft. At PA = 5,000 ft and OAT 25 °C, ISA = 5 °C, DA = 5,000 + 120 × 20 = 7,400 ft.

How do I get pressure altitude?

PA = indicated altitude + (29.92 − local altimeter setting) × 1,000. At a 4,500 ft field with altimeter 29.62, PA = 4,500 + 300 = 4,800 ft. Or set the altimeter to 29.92 momentarily and read PA directly.

Why does density altitude matter so much?

Engine HP, prop thrust, and wing lift all drop with thinner air. Takeoff roll grows roughly 10% per 1,000 ft of DA, climb rate falls ≈ 10%, and service ceiling drops. DA accidents are a leading cause of summer GA crashes.

What about humidity?

Humid air is less dense (water vapor is lighter than dry air). Standard DA formulas omit humidity, but real DA on a hot humid day may be 200–500 ft higher than the dry calculation. Educational only - use POH performance charts for actual flight.

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Aviation & Marine Disclaimer: Educational only. Not for flight or navigation operations.

This calculator is for educational and informational purposes only and is not a substitute for official flight or navigation planning. Always use current performance charts, an approved POH/AFM, certified navigation tools, official charts, and current weather sources, and follow all applicable FAA, ICAO, USCG, and other regulatory guidance. Do not rely on this tool for real-world flight, marine, or trailering decisions. Verify all results independently before operating any aircraft or vessel.

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How This Calculator Works

Density altitude (DA) is pressure altitude corrected for non-standard temperature - it is the altitude at which the International Standard Atmosphere would have the same air density as the conditions you are flying in. Aircraft (wings, propellers, normally-aspirated engines) all behave as if they were at this altitude: a 95°F day at a 5,000-ft airport can give a density altitude over 8,500 ft, with dramatically degraded takeoff and climb performance. Every accident summary of high-DA crashes - overloaded planes failing to clear obstacles on hot summer days - traces back to a pilot who did not run this number. This calculator takes field elevation, altimeter setting (Kollsman), and OAT, computes pressure altitude, the ISA temperature at that pressure altitude, and the DA. Use for planning study; verify performance against the POH/AFM takeoff and climb charts.

Formula / Method

Pressure altitude: PA = field elevation + (29.92 − altimeter) × 1000 (ft). The ISA temperature at PA is T_ISA = 15°C − 2°C × (PA ÷ 1000). The Koch-chart approximation for density altitude is DA ≈ PA + 120 × (OAT − T_ISA) (DA in feet, temperatures in °C). The constant 120 captures ≈120 ft per °C above standard. A more precise treatment uses virtual temperature corrected for humidity, but the simple form is well within charting accuracy for most light-aircraft planning.

Worked Example

Mountain airfield: field elevation 5,000 ft, altimeter 29.92 inHg, OAT 25°C. PA = 5,000 + (29.92 − 29.92) × 1000 = 5,000 ft. T_ISA at 5,000 ft = 15 − 2 × 5 = 5°C. DA = 5,000 + 120 × (25 − 5) = 5,000 + 2,400 = 7,400 ft. The aircraft will perform like it is sitting at 7,400 ft on a standard day - expect roughly 25–30% longer takeoff roll vs. sea level and a markedly weaker rate of climb. Add a 200 lb safety margin to your weight figure and consult the POH high-DA takeoff chart before going.

Common Mistakes

Treating "altitude" from the GPS as pressure altitude - GPS gives geometric (true) altitude, which differs from PA whenever the altimeter is not 29.92. Forgetting to convert Fahrenheit to Celsius before plugging into the formula (a 95°F input read as Celsius would give absurd DA). Ignoring humidity on a hot, muggy day - the simple formula understates DA by a few hundred feet in extreme dewpoint conditions. Assuming DA only matters "in the mountains" - a hot summer day at a sea-level field can still push DA above 2,000 ft and cost meaningful runway. Using DA to set the altimeter - DA is a performance figure only; never crank it into the Kollsman window.

Tips & FAQ

How does DA affect performance? Roughly: takeoff distance increases ≈10% per 1,000 ft DA; rate of climb drops ≈7–10% per 1,000 ft; service ceiling drops by the same amount you climbed in DA. Why 120 ft / °C? It is a chart-fit constant valid up to about 8,000 ft DA; above that the ratio changes and you should use a Koch chart or POH performance section directly. What altimeter setting do I use? The current local altimeter from ATIS/AWOS/METAR - not 29.92 unless you are flying flight-levels above transition altitude. Disclaimer: this calculator is a planning aid; always cross-check against your POH/AFM takeoff and climb performance charts and current weather.

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