How to Calculate Density Altitude, Step by Step

Density altitude is the altitude the airplane “feels”: the height in the standard atmosphere where the air is as thin as it is today. Hot days, high airports and low pressure all raise it, which means a longer takeoff roll and a slower climb.

For learning only. This lesson is for studying and ground-school work, not for planning a real flight. Use official weather, charts and flight-planning tools, and the numbers in your aircraft’s handbook (POH).

Try it with your own problem ↓

The steps

Find the pressure altitude: field elevation + (29.92 − altimeter setting) × 1,000 ft. (Or set 29.92 in the altimeter and read it.)
Find the standard (ISA) temperature at that altitude: 15 °C minus 2 °C for every 1,000 ft.
Subtract: outside air temperature (OAT) − ISA temperature. Positive means warmer than standard.
Density altitude ≈ pressure altitude + 120 ft × (OAT − ISA).
Check the sense (warmer than standard gives a density altitude above the pressure altitude), then use the handbook’s performance charts.

A worked example

An airport sits at 3,500 ft. The altimeter setting is 30.12 inHg and the temperature is 25 °C. Find the density altitude.

Pressure difference: 29.92 − 30.12 = −0.20 inHg, and −0.20 × 1,000 = −200 ft.
Pressure altitude: 3,500 − 200 = 3,300 ft.
ISA temperature at 3,300 ft: 15 − 2 × 3.3 = 15 − 6.6 = 8.4 °C.
OAT − ISA: 25 − 8.4 = 16.6 °C warmer than standard.
Density altitude: 3,300 + 120 × 16.6 = 3,300 + 1,992 = 5,292 ft.

The density altitude is about 5,292 ft: the airplane will perform as if the field were nearly 1,800 ft higher. (The full standard-atmosphere formula gives about 5,193 ft.)

Why it works

Air gets thinner as you climb, and it also gets thinner as it warms, because warm air expands. The standard atmosphere is a table that pairs each altitude with a standard pressure and temperature. Pressure altitude fixes the pressure; then each degree warmer than standard thins the air about as much as climbing another 120 feet would.

Try it with your own problem

Open The Flight Computer (free) with this example already filled in. It shows every step, and you can type over it with your own numbers.

Work it out: elevation 3,500 ft, altimeter 30.12, 25 °COpens The Flight Computer with this example

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More aviation math, every step written out: open The Flight Computer.

Common mistakes

Getting the altimeter correction backward. A setting above 29.92 means higher pressure than standard, so the pressure altitude is below the field elevation.
Using the sea-level ISA. The standard temperature drops 2 °C per 1,000 ft: at 3,300 ft it is 8.4 °C, not 15 °C.
Using °F. The 120-ft rule is per degree Celsius. Convert first: °C = (°F − 32) × 5/9.
Starting from the elevation. Correct for pressure first, then for temperature.

Questions people ask

What is the density altitude at 5,000 ft and 30 °C?

ISA at 5,000 ft is 15 − 10 = 5 °C, so it is 25 °C warmer than standard: 5,000 + 120 × 25 = 8,000 ft.

Why does density altitude matter?

Thin air gives the engine less power, the propeller less bite and the wings less lift. A takeoff that is easy at sea level can need much more runway on a hot day at a high airport.

Is the 120-ft rule exact?

No. It is the rule of thumb in the FAA’s handbooks and is close for light airplanes. Flight computers and performance charts use the full standard-atmosphere formula.

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