Density Altitude Calculator: How to Find It

A density altitude calculator converts your current pressure altitude and outside air temperature into the altitude at which the air density actually matches. You enter your station pressure (or altimeter setting and field elevation), temperature, and dew point, and the calculator returns density altitude in feet or meters. This value tells pilots how an aircraft will perform, because lower air density reduces lift, thrust, and engine power.

Formula

The standard formula for density altitude (in feet) is:

Density Altitude = Pressure Altitude + 118.8 × (OAT − ISA Temperature at that altitude)

Where:

• Pressure Altitude (ft): the altitude shown on an altimeter set to 29.92 inHg (1013.25 hPa). It is the vertical distance above the standard datum plane.
• OAT (°F or °C): the outside air temperature at your location and altitude.
• ISA Temperature (°C): the International Standard Atmosphere temperature at that altitude, calculated as 15°C − 0.00198 × pressure altitude (in feet) or equivalently 59°F − 0.00356 × pressure altitude (in feet).

The constant 118.8 (when using °F) comes from the standard lapse rate and the gas law; if you use Celsius, the constant is 118.8 × (5/9) ≈ 66.0.

Worked Example

Using the calculator defaults: pressure altitude = 5,000 ft, OAT = 70°F, dew point = 50°F.

1. Find the ISA temperature at 5,000 ft: 59 − 0.00356 × 5000 = 59 − 17.8 = 41.2°F.
2. Subtract ISA from OAT: 70 − 41.2 = 28.8°F.
3. Multiply by 118.8: 28.8 × 118.8 = 3421.44 ft.
4. Add to pressure altitude: 5000 + 3421.44 = 8421.44 ft.

The calculator rounds to 8421 ft. Note that the dew point does not appear in this simple formula; it is used in more precise density altitude calculations that account for water vapor, but the effect is usually small (tens of feet).

Why Density Altitude Matters

Density altitude is not a physical altitude you can see; it is a performance number. When density altitude is high, the air is less dense, which means:

• An engine takes in less oxygen per cylinder, so it produces less power (normally aspirated engines lose roughly 3% of power per 1,000 ft increase in density altitude).
• A propeller or rotor is less efficient because it moves less air per revolution.
• A wing generates less lift at a given true airspeed, so takeoff and landing distances increase.
• Climb rate decreases, and the service ceiling is effectively lower.

For these reasons, density altitude is a standard preflight calculation for pilots, especially at high elevations or on hot days. A runway at 5,000 ft elevation on a 100°F day can have a density altitude of 8,000 ft or more, which can make a marginal takeoff impossible.

How to Get Pressure Altitude

You can obtain pressure altitude in two ways:

• Set your altimeter to 29.92 inHg (1013.25 hPa) and read the indicated altitude. That reading is pressure altitude.
• Calculate it from field elevation and altimeter setting: Pressure Altitude = Field Elevation + (29.92 − Altimeter Setting) × 1000. For example, if field elevation is 2,000 ft and the altimeter setting is 29.72, pressure altitude = 2000 + (0.20 × 1000) = 2,200 ft.

In the calculator above, you can enter pressure altitude directly. If you only have field elevation and altimeter setting, use the second method first.

Common Mistakes and Misconceptions

• Using field elevation instead of pressure altitude. Field elevation is the actual height above sea level; pressure altitude is the height in the standard atmosphere. On a high-pressure day, pressure altitude is lower than field elevation; on a low-pressure day, it is higher.
• Mixing units. The 118.8 constant works with °F. If you use Celsius, you must convert or use the 66.0 constant. The calculator above expects °F for both OAT and the ISA formula.
• Ignoring humidity. The simple formula ignores dew point. Humid air is less dense than dry air, so the true density altitude is slightly higher. The effect is usually less than 100–200 ft even in extreme humidity, but for precise work you can use a formula that includes vapor pressure.
• Thinking density altitude is the same as true altitude. True altitude is the actual height above sea level; density altitude is a performance indicator. They can differ by thousands of feet.

Limits of the Simple Formula

The formula used here is a standard approximation that works well for typical aviation conditions (pressure altitude up to about 20,000 ft and temperatures within ±50°F of ISA). It assumes a dry atmosphere and a standard lapse rate. For extreme altitudes (e.g., mountain flying above 15,000 ft) or very cold temperatures, more sophisticated models may be needed. Also, the formula does not account for wind, which affects takeoff and landing performance separately.

For a more precise density altitude, you can use the following equation that includes dew point (in °C):

DA = PA + 120 × (OAT − ISA) + 0.5 × (OAT − ISA)² + (dew point correction)

But the simple 118.8 formula is accurate enough for most preflight planning.

How to Use the Calculator

1. Determine your pressure altitude using one of the methods above.
2. Enter the outside air temperature in °F (or convert from °C).
3. Enter the dew point if you have it; the calculator currently ignores it, but you can use it with a more complete formula if needed.
4. Read the density altitude in feet and meters. The first output is the headline answer.

For example, if you are at a mountain airport with pressure altitude 7,000 ft and temperature 80°F, the calculator gives a density altitude of about 9,300 ft. That means your aircraft performs as if it were at 9,300 ft on a standard day.

Frequently Asked Questions

What is the difference between density altitude and pressure altitude?

Pressure altitude is the altitude in the standard atmosphere where the pressure equals your current pressure. Density altitude is the altitude in the standard atmosphere where the air density equals your current density. Pressure altitude is a pressure measurement; density altitude accounts for temperature (and humidity).

Why does density altitude increase on a hot day?

Hot air expands, so the same number of molecules occupies a larger volume. That means fewer molecules per cubic foot, so air density decreases. To find air with the same low density in the standard atmosphere, you have to go higher, so density altitude increases.

How does humidity affect density altitude?

Water vapor molecules are lighter than the nitrogen and oxygen molecules they replace. So humid air is less dense than dry air at the same temperature and pressure. This makes density altitude slightly higher on humid days, but the effect is small—usually less than 100 ft.

Can density altitude be negative?

Yes. On a cold day at a low elevation, the air can be denser than the standard atmosphere, so density altitude can be below sea level (negative feet). This is common in winter at airports near sea level.

Do I need to adjust for humidity in the calculator?

The simple formula does not use dew point, but you can use a more complete formula if you need extreme precision. For most preflight calculations, the difference is negligible.

What is a typical density altitude at a 5,000 ft airport on a summer day?

At 5,000 ft pressure altitude and 85°F, density altitude is roughly 7,500 ft. On a 100°F day, it can exceed 9,000 ft. That is why high-elevation airports have published performance charts that account for density altitude.

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in alt | Pressure altitude | ft | 5000 | 0
in oat | Outside air temperature | °F | 70 | -100
in dew | Dew point | °F | 50 | -100
out da | Density altitude | ft | alt + 118.8 * (oat - (59 - 0.00356 * alt / 0.3048)) | 0
out da_m | Density altitude (m) | m | da * 0.3048 | 0