What is the formula for density altitude?

The standard pilot formula is: **Density Altitude = Pressure Altitude + 120 × (OAT − ISA Temperature)**, where OAT is outside air temperature in °C and ISA temperature is 15°C at sea level decreasing 2°C per 1,000 ft. Pressure altitude itself is computed as field elevation + (29.92 − altimeter setting) × 1,000. The 120 ft per °C correction is an approximation that's accurate enough for preflight planning and what most DPEs expect to see on a checkride.

How do I calculate pressure altitude?

Two ways. **In the cockpit:** set 29.92 in your altimeter's Kollsman window and read the indicated altitude — that's pressure altitude directly. **On paper:** PA = Field Elevation + (29.92 − Altimeter Setting) × 1,000. So if you're at a 2,000 ft field with an altimeter setting of 29.72, pressure altitude is 2,000 + (29.92 − 29.72) × 1,000 = 2,200 ft. Lower altimeter settings produce higher pressure altitudes.

At what density altitude does aircraft performance become a problem?

There's no single threshold, but for a normally-aspirated trainer like a Cessna 172, performance degrades noticeably above about **3,000 ft DA** and becomes critical above **6,000–8,000 ft DA**. By 10,000 ft DA you're approaching service ceiling. The right question isn't "is DA high?" — it's "what does the POH chart say my takeoff distance and climb rate are at this DA, and is that compatible with the runway and obstacles ahead?"

Does humidity affect density altitude?

Yes. Water vapor is less dense than dry air, so humid air has lower density at the same temperature and pressure. Standard E6B and POH density altitude calculations ignore humidity, but on a hot, humid day you can add roughly **500–1,000 ft** of effective density altitude beyond what your calculation shows. Treat published performance numbers as best-case in high-humidity conditions, especially in the Southeast US in summer.

Why is density altitude higher than pressure altitude on hot days?

Because hot air is less dense than cold air. Density altitude tells you the equivalent altitude in the standard atmosphere where the air would have the same density as the air you're in. When the temperature is above ISA, the actual air is thinner than standard for that pressure altitude, so its equivalent altitude in the standard atmosphere is higher — that's your density altitude. Every 1°C above standard adds about 120 ft of density altitude.

Is density altitude required preflight planning?

Effectively yes. [FAR 91.103](/far/91-103) requires the pilot in command to become familiar with all available information concerning the flight, including **runway lengths and takeoff/landing distance information**. Those POH numbers are computed against density altitude, so you can't comply with 91.103 without computing DA. [FAR 91.9](/far/91-9) separately requires operating within POH limitations, which are also DA-dependent (e.g., service ceiling, maximum operating altitude).

How does density altitude affect indicated vs true airspeed?

Indicated airspeed at rotation and approach stays the same regardless of density altitude — the wing stalls at the same indicated speed. But **true airspeed increases by roughly 2% per 1,000 ft of density altitude**. So at a DA of 8,000 ft, your 60-knot indicated rotation speed is actually about 69 knots true. That's why takeoff ground roll and landing distance grow at high DA: you're moving faster across the ground for the same indicated airspeed.

What's the difference between an E6B and an electronic density altitude calculator?

Functionally they produce the same answer. A manual E6B uses the same physical formula encoded in slide-rule form; an electronic E6B or app does the math digitally. DPEs may ask you to demonstrate either method, and many require you to know the underlying formula regardless of tool. The advantage of doing it by hand once is that you internalize the relationship between pressure altitude, temperature deviation, and the resulting DA — knowledge an app can't give you.

Density Altitude Calculator & Formula

What density altitude actually is

Density altitude is pressure altitude corrected for non-standard temperature. It's the altitude your airplane thinks it's flying at — the altitude in the standard atmosphere that has the same air density as the air you're actually sitting in. When the air gets thinner (hot, high, humid), your wings produce less lift, your prop bites less air, and your normally-aspirated engine makes less power. The number that captures all of this in one figure is density altitude.

This matters because aircraft performance charts in your POH are built around it. Takeoff distance, climb rate, service ceiling, and true airspeed all key off density altitude. FAR 91.103 makes preflight performance computation a legal requirement for every flight — and density altitude is the input.

The two-step calculation

The FAA expects you to compute density altitude in two steps: pressure altitude first, then correct for temperature deviation from standard.

Step 1 — Pressure altitude

Pressure Altitude (PA) = Field Elevation + (29.92 − Altimeter Setting) × 1,000

If the altimeter setting is below 29.92, pressure altitude is higher than field elevation (low pressure = thin air). If it's above 29.92, pressure altitude is lower. The quick alternative on the ramp: set 29.92 in your Kollsman window and read pressure altitude directly off the altimeter.

Step 2 — Correct for temperature

Density Altitude (DA) ≈ PA + 120 × (OAT − ISA Temp)

Where:

OAT = outside air temperature in °C
ISA Temp = standard temperature at that pressure altitude
ISA at sea level = 15°C, decreasing 2°C per 1,000 ft

The 120 ft per °C factor is the working approximation pilots use; an E6B or electronic calculator will give you a slightly more precise answer, but the difference is rarely operationally meaningful.

Worked example — a hot day at a mountain airport

Let's compute density altitude for a Cessna 172 sitting on the ramp at Leadville, Colorado (KLXV) on a summer afternoon.

Field elevation: 9,934 ft
Altimeter setting: 30.12 in Hg
OAT: 25°C

Pressure altitude:

PA = 9,934 + (29.92 − 30.12) × 1,000
PA = 9,934 + (−200)
PA = 9,734 ft

ISA temperature at 9,734 ft:

Field Elev	Altimeter	OAT	Pressure Alt	Density Alt
Sea level	29.92	15°C	0 ft	0 ft (ISA)
1,500 ft	29.92	30°C	1,500 ft	~3,300 ft
5,000 ft (KAPA)	30.00	32°C	4,920 ft	~8,200 ft
7,000 ft	29.80	30°C	7,120 ft	~10,800 ft
9,934 ft (KLXV)	30.12	25°C	9,734 ft	~13,300 ft

Density Altitude Calculator and How to Compute It

What density altitude actually is

The two-step calculation

Step 1 — Pressure altitude

Step 2 — Correct for temperature

Worked example — a hot day at a mountain airport

Quick-reference table — typical DA values

Why it matters: real performance penalties

Humidity — the factor pilots forget

How to compute DA in the cockpit

Density altitude on the checkride

How GroundScholar helps with this

Key takeaways