Density Altitude Calculator — Pressure Altitude, ISA Temperature & DA Calc

Q: How do I calculate density altitude?

First find pressure altitude from field elevation and the altimeter setting. Then compute the air density from that pressure and the outside air temperature (using virtual temperature if you have a dewpoint). Density altitude is the height in the standard atmosphere that has the same density. This calculator uses the ICAO Standard Atmosphere formula; the common rule of thumb is DA ≈ pressure altitude + 120 × (OAT − ISA temperature).

Q: What is the difference between pressure altitude and density altitude?

Pressure altitude is the height in the standard atmosphere that has the same pressure as the air you are in. Density altitude is the height that has the same density. Temperature and humidity change density, so density altitude is usually higher than pressure altitude on a hot day and is the number that describes airplane performance.

Q: What is ISA temperature and ISA deviation?

ISA temperature is the standard-atmosphere temperature at your pressure altitude, starting at 15°C at sea level and falling about 2°C per 1,000 feet. ISA deviation is the difference between the actual outside air temperature and that standard value (ISA + or −).

Q: How does high density altitude affect takeoff?

High density altitude means thinner air, so the takeoff roll is longer and the climb is weaker. A common rule of thumb is about 10% more takeoff roll and about 8% less climb rate per 1,000 feet of density altitude versus sea-level standard conditions. Always use the performance charts in your POH or AFM.

What is density altitude?

Density altitude is the altitude in the International Standard Atmosphere (ISA) at which the air has the same density as the air you are actually flying in. Airplanes, engines, and propellers produce less performance when the air is thin. Heat, high elevation, low pressure, and (to a lesser extent) humidity all raise density altitude, so a 5,000 ft airport on a 30°C day can “feel” like roughly 8,000 ft to the airplane.

Why it matters for takeoff and climb

Higher density altitude means a longer takeoff roll, a shallower climb, a higher true airspeed for the same indicated airspeed, and less excess power. That combination is what makes hot-and-high departures unforgiving, especially from short or high-elevation runways. Calculate density altitude before you launch, then read the POH charts for that density altitude — not for field elevation alone.

How this calculator works

The primary results use the ICAO Standard Atmosphere (ICAO Doc 7488 / ISO 2533) tropospheric model, valid below 11 km (36,089 ft).

Station pressure is the altimeter setting (QNH) reduced to field elevation with the ISA pressure-height relation the altimeter assumes:
p_s = QNH × (1 − L·h / T₀)^{g / (R L)}
Pressure altitude is the ISA height that has that same pressure:
PA = (T₀ / L) × (1 − (p_s / p₀)^{R L / g})
Density altitude (dry air) is the ISA height that has the same density. With density ratio σ = (p_s / p₀) × (T₀ / T):
DA = (T₀ / L) × (1 − σ^{1 / (g/(R L) − 1)})
Humidity-corrected density altitude uses virtual temperature in place of OAT, so moist air (which is less dense) produces a slightly higher DA. Vapor pressure comes from the Bolton (1980) / Magnus-Tetens formula, and T_v = T / (1 − 0.378 · e / p_s).

Constants: T₀ = 288.15 K, L = 0.0065 K/m, p₀ = 1013.25 hPa, g = 9.80665 m/s², R = 287.05287 J/(kg·K). In feet, T₀/L is 145,442.16 ft, and the density exponent is about 0.234969.

The common FAA / AOPA rule of thumb from the Pilot’s Handbook of Aeronautical Knowledge, DA ≈ PA + 120 × (OAT − ISA) with ISA = 15°C − 1.98°C per 1,000 ft of pressure altitude, is a useful mental check. On the worked example below it reads about 7,989 ft, a little higher than the ICAO density formula.

Worked example

A 5,000 ft field, altimeter 29.92 inHg, OAT 30°C, no dewpoint:

Pressure altitude = 5,000 ft (29.92 inHg is treated as the ICAO standard 29.92126 inHg / 1013.25 hPa).
ISA temperature at that altitude ≈ 5.1°C, so the ISA deviation is about +24.9°C.
ICAO density altitude ≈ 7,801 ft (often rounded to 7,800 ft). Air density is about 0.969 kg/m³, or 79% of sea-level ISA.
The 120 ft/°C rule of thumb gives ≈ 7,989 ft, which is the familiar “about 8,000 ft” check.
With a 20°C dewpoint, virtual temperature is about 33.2°C and humidity-corrected DA rises to about 8,141 ft (+340 ft).

How to use it

Enter the field elevation in feet or metres.
Enter the current altimeter setting in inches of mercury or hectopascals. The two fields stay in sync.
Enter OAT in °C or °F, or enter ISA deviation (ISA +/−) instead. The other field updates automatically. Last-edited wins: if you set ISA +15 and then change elevation, OAT follows so the deviation stays +15.
Optionally enter dewpoint to see the humidity correction. On a dry desert day the correction is small; on a humid afternoon it can add a few hundred feet.
Read pressure altitude and density altitude, then open the POH performance charts. Treat the takeoff and climb percentages on this page as a gut check only.

The performance rule of thumb

The bars on this page assume a 10% increase in takeoff roll and an 8% decrease in climb rate for each 1,000 ft of density altitude, relative to sea-level standard conditions. Those percentages are in the same ballpark as many light-GA POH charts, but they are not aircraft-specific, they do not include weight, wind, slope, or surface, and they become less trustworthy at very high density altitude. If the estimate looks ugly, believe the warning — then go to the book.

How to calculate density altitude

Start with field elevation and the altimeter setting to get pressure altitude. Then use the outside air temperature — and dewpoint, if you have it — to find the actual air density. Density altitude is the altitude in the standard atmosphere with that same density. This page uses the ICAO formula; a quick E6B-style check is DA ≈ PA + 120 × (OAT − ISA), which usually reads a little higher on a hot day.

Frequently asked questions

How do I calculate density altitude?

Convert elevation and altimeter setting to pressure altitude, then correct for temperature (and humidity, if known) to get density altitude. The calculator above does that with the ICAO Standard Atmosphere; the 120 ft/°C rule of thumb is a useful mental check.

What is the difference between pressure altitude and density altitude?

Pressure altitude matches the standard atmosphere at the same pressure. Density altitude matches it at the same density. Heat and humidity raise density altitude above pressure altitude, which is why a 5,000 ft airport on a 30°C day can perform like roughly 8,000 ft.

What is ISA temperature and ISA deviation?

ISA temperature is 15°C at sea level, decreasing about 2°C per 1,000 ft of pressure altitude. ISA deviation (ISA +/−) is how far the actual OAT sits from that standard value.

How does high density altitude affect takeoff?

Thinner air means a longer takeoff roll and a weaker climb. Treat the percentages on this page as a gut check only, then go to the POH performance charts for your airplane, weight, and runway.