How is density altitude calculated?

SkyStudy uses the standard rule: density altitude = pressure altitude + 120 ft for every °C the outside air temperature is above the ISA temperature at that level.

What is the density altitude formula?

Density altitude ≈ pressure altitude + 120 × (OAT − ISA temperature), where the ISA temperature at a level is 15 − 1.98 × (pressure altitude ÷ 1000) °C, often rounded to 2 °C per 1000 ft for mental maths.

Why does density altitude matter?

It is the altitude the aircraft performs at. High density altitude, hot, high airfields, means longer take-off and landing rolls, reduced climb, and lower engine and propeller efficiency.

Do I need pressure altitude first?

Yes. Calculate pressure altitude from your elevation and QNH first, then enter that here with the outside air temperature.

Does humidity affect density altitude?

Slightly. Water vapour is lighter than dry air, so humid air is a little less dense and the true density altitude is a little higher than the dry-air rule suggests. The ATPL formula ignores humidity, and so do the exam questions, but real-world performance planning treats a hot, humid day as the worst combination.

Performance maths

Density altitude calculator.

See the altitude your aircraft really feels on a hot or high day, the single biggest driver of take-off, climb, and landing performance.

Density altitude

Enter the pressure altitude and the outside air temperature. SkyStudy uses the 120 ft per °C of ISA deviation rule, high, hot and humid air pushes density altitude up and performance down.

Pressure altitude (ft)

Outside air temp (°C)

Enter a pressure altitude and temperature to see the density altitude.

In the EASA ATPL exam

Density altitude is a staple of ATPL Performance (subject 032), where it sits behind almost every take-off and landing distance question, and it returns in Meteorology (050) whenever a question asks how temperature changes air density. Examiners rarely hand you the density altitude directly: they give an airfield elevation, a QNH and an outside air temperature, and expect you to build the chain yourself, elevation to pressure altitude, pressure altitude to ISA deviation, deviation to density altitude.

Exam-style example

An airfield has a pressure altitude of 2,000 ft and the outside air temperature is +31 °C. What is the density altitude?

Find the ISA temperature at 2,000 ft: 15 − (2 × 2) = +11 °C.
Find the ISA deviation: +31 − 11 = ISA +20.
Apply the rule of thumb: 120 ft × 20 = 2,400 ft above pressure altitude.

Density altitude = 2,000 + 2,400 = 4,400 ft.

Common trap: Comparing the OAT with +15 °C instead of with the ISA temperature at the level. The standard temperature falls about 2 °C per 1,000 ft, so at 2,000 ft the reference is +11 °C, not +15 °C. Answer options built on that wrong reference are usually waiting in the question.

ATPL Performance guide Pressure altitude calculator ISA deviation calculator

How to calculate density altitude

The definition, the formula, a worked example and how it shows up in the ATPL exams.

What density altitude is

Density altitude is the pressure altitude corrected for temperature. More precisely, it is the altitude in the International Standard Atmosphere at which the air density matches the density you currently have. It is the altitude your wing, engine and propeller actually 'feel', regardless of what the field elevation says.

Warm air is less dense than standard air, so on a hot day the aircraft behaves as though it were much higher than its real elevation. This is the single biggest driver of take-off, climb and landing performance, and the reason 'hot and high' airfields are demanding.

The 120 ft per °C rule and the formula

The ATPL rule of thumb is: density altitude ≈ pressure altitude + 120 ft for every degree Celsius the air is warmer than ISA at that level. First find the ISA temperature for the pressure altitude (15 °C at sea level, falling about 2 °C per 1000 ft), then compare it with the actual outside air temperature to get the ISA deviation, then apply 120 ft per degree. SkyStudy computes the ISA temperature with the precise 1.98 °C per 1,000 ft lapse rate, so its result can differ by a few feet from a mental-maths estimate that rounds to 2 °C.

If the air is colder than standard the density altitude drops below the pressure altitude and performance improves. The effect is symmetric: 120 ft per °C either way.

Worked example

Pressure altitude 3,000 ft, outside air temperature +25 °C. The ISA temperature at 3,000 ft is 15 − (2 × 3) = 9 °C. The air is therefore 25 − 9 = 16 °C warmer than standard, an ISA+16 deviation. Apply the rule: 120 × 16 = 1,920 ft. Density altitude = 3,000 + 1,920 = 4,920 ft.

So a runway physically at 3,000 ft is asking the aircraft to perform as though it were nearly 5,000 ft up. That is why take-off and climb charts can look alarming on a hot afternoon at a high airfield.

Where it shows up in the ATPL exams

Density altitude is core to ATPL Performance and recurs in Meteorology and Principles of Flight. To get there you first need pressure altitude (use the pressure altitude calculator) and the ISA deviation (use the ISA deviation calculator). Together they tell the full story of why the same aircraft, same weight and same runway can be safe in the morning and marginal in the afternoon heat.

Good to know

Background and tips for getting the most out of this calculator.

Hot and high

Warm air is thinner. Every °C above standard adds about 120 ft to the density altitude and erodes performance margins.

From the top

Use the pressure altitude calculator first, then bring that value here with the OAT for the full picture.

Plan conservatively

Performance charts assume the density altitude, not your field elevation. On a hot day the difference can be thousands of feet.

Frequently asked questions

Tap to read the full background and answers.

How is density altitude calculated?: SkyStudy uses the standard rule: density altitude = pressure altitude + 120 ft for every °C the outside air temperature is above the ISA temperature at that level.
What is the density altitude formula?: Density altitude ≈ pressure altitude + 120 × (OAT − ISA temperature), where the ISA temperature at a level is 15 − 1.98 × (pressure altitude ÷ 1000) °C, often rounded to 2 °C per 1000 ft for mental maths.
Why does density altitude matter?: It is the altitude the aircraft performs at. High density altitude, hot, high airfields, means longer take-off and landing rolls, reduced climb, and lower engine and propeller efficiency.
Do I need pressure altitude first?: Yes. Calculate pressure altitude from your elevation and QNH first, then enter that here with the outside air temperature.
Does humidity affect density altitude?: Slightly. Water vapour is lighter than dry air, so humid air is a little less dense and the true density altitude is a little higher than the dry-air rule suggests. The ATPL formula ignores humidity, and so do the exam questions, but real-world performance planning treats a hot, humid day as the worst combination.

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