The core concept

“Density altitude is pressure altitude corrected for non-standard temperature. In simple terms: it’s the altitude the airplane thinks it is flying at.”

High density altitude — thin air

Bad performance: longer takeoff roll, slower climb rate.

Low density altitude — dense air

Great performance: short roll, strong climb.

Decode the variables — the legend

Every acronym below — the right column tracks your numbers live

FE	Field elevation. Height of the airport above sea level, in feet.	–	input
AS	Altimeter setting. Local barometric pressure, inches of mercury.	–	input
OAT	Outside air temperature. Actual temperature at the airport, °C.	–	input
PA	Pressure altitude. Altitude when the altimeter is set to 29.92.	–	step 1
ISA	Standard temperature. 15 °C at sea level, minus 2 °C per 1,000 ft.	–	step 2
DA	Density altitude. The final calculated performance altitude.	–	step 3

Step by step, on your numbers

Enter the three inputs

Field Elevation (FE) — ft

From the FAA Airport Data portal (ADIP) — the one input not in the METAR.

Altimeter Setting (AS) — inHg

The METAR A group: A2972 = 29.72 inHg.

Outside Air Temp (OAT) — °C

First number of the METAR temperature group: 30/21 = 30 °C.

Pull the current METAR for any airport at aviationweather.gov, or decode it with the METAR decoder first if you want it translated.

Step 1 — Pressure Altitude (PA)

WHY WE DO THISThe standard atmosphere assumes 29.92 inHg at sea level, and pressure changes about 1 inHg per 1,000 ft — so this converts today’s pressure difference into feet and moves the field elevation by that amount.

PA = FE + (29.92 − AS) × 1,000

Every 0.01 inHg = 10 feet. A setting above 29.92 pushes PA below field elevation, and vice versa.

Step 2 — Standard Temperature (ISA)

WHY WE DO THISOn a standard day the air is 15 °C at sea level and cools 2 °C per 1,000 ft. This finds the baseline your actual temperature gets compared against in step 3.

ISA = 15 − 2 × (PA ÷ 1,000)

15 °C at sea level, cooling 2 °C per 1,000 feet.

Step 3 — Density Altitude (DA)

WHY WE DO THISEvery 1 °C warmer than standard thins the air enough to make the airplane behave about 120 ft higher, so the difference is multiplied by 120 and added to pressure altitude.

DA = PA + 120 × (OAT − ISA)

120 feet per °C above standard; subtract 120 per degree below it.

The takeaway check

The airport elevation

–

The calculated DA

–

Density altitude chart

The same answer, graphically. The solid marker lines trace the exact path you would draw on the printed FAA chart.

How to read it

Enter at the bottom at your outside air temperature and rise to your pressure-altitude diagonal — the red dashed diagonal is your exact PA, interpolated between the printed lines. From that intersection, read straight across to the left axis. That is your density altitude.

The steep dashed line is standard temperature (ISA): whenever your point sits right of it, the air is warmer than standard and DA exceeds PA. The chart is drawn from the same 120 ft/°C rule as the steps above, so the graphical answer and the arithmetic always agree.

The printed FAA chart uses the full standard-atmosphere model and can differ by roughly 100–200 feet — on test day, work the printed chart as procedure and let the mental math confirm it. Everything here assumes dry air: on humid days (dew point 20 °C or higher), true density altitude runs a few hundred feet higher than any dry-air method shows.