The 6 Types of Altitude Every Pilot Should Know

Pilots work with six types of altitude: indicated altitude (what the altimeter reads), calibrated altitude (indicated corrected for instrument error), pressure altitude (height above the 29.92 inHg standard datum), density altitude (pressure altitude corrected for temperature), true altitude (actual height above mean sea level), and absolute altitude (height above the ground).

The six altitudes at a glance

Indicated altitude: what you read

Indicated altitude is simply the number on the face of the altimeter when the current local altimeter setting is dialed into the Kollsman window. It’s the altitude you fly, the altitude ATC assigns, and the altitude that keeps you separated from other traffic below 18,000 ft — because everyone nearby is using the same setting and therefore making the same small errors together.

Example. Holding short at a field that sits at 427 ft MSL with the ATIS altimeter of 30.05 set, the needle should park at about 427 ft. More than roughly 75 ft off, and the instrument needs a shop visit before any IFR flying.

Calibrated altitude: indicated, minus the lies

No altimeter is perfect. The mechanism has small internal error, and the static port feeding it sits in airflow that doesn’t always match the true outside pressure — position error, which varies with airspeed and configuration. Calibrated altitude is indicated altitude corrected for both, via the correction card or POH table.

In practice the correction is tens of feet — but it is the legally tested quantity (IFR altimeters are recertified every 24 calendar months) and the correct starting point for computing true altitude on a flight computer.

Pressure altitude: your height above the standard datum

Set 29.92 in the Kollsman window and the altimeter measures your height above the standard datum plane — the theoretical level where pressure equals 29.92 inHg. That reading is pressure altitude: the currency of performance charts, flight levels, and every density-altitude calculation. You don’t need to touch the knob, though:

Worked example. Field elevation 4,200 ft, altimeter setting 30.12. PA = 4,200 + (29.92 − 30.12) × 1,000 = 4,200 − 200 = 4,000 ft. On a high-pressure day your pressure altitude sits below field elevation; on a low-pressure day it climbs above it. The pressure altitude calculator does this in one step, and at or above 18,000 ft everyone sets 29.92 permanently — a flight level is just pressure altitude in hundreds of feet (FL350 = 35,000 ft PA).

Density altitude: the performance altitude

Pressure altitude tells you where you sit on the standard pressure ladder, but your wing, propeller, and engine respond to air density — and density also depends on temperature. Density altitude is pressure altitude corrected for non-standard temperature. Standard (ISA) temperature is 15 °C at sea level, falling about 2 °C per 1,000 ft, and each degree above standard adds roughly 120 ft:

Worked example. Continue from above: PA is 4,000 ft, so ISA temperature there is 15 − 8 = 7 °C. The OAT is a warm 27 °C — 20 °C above standard — so DA ≈ 4,000 + 120 × 20 = 6,400 ft. Your airplane takes off, climbs, and lands as if it were at 6,400 ft on a standard day. This is the one altitude that predicts performance, which is why it gets its own density altitude calculator and a full pilot’s guide to density altitude on this site.

True altitude: where you actually are — and the cold-weather trap

True altitude is your real height above mean sea level — the number that terrain and obstacles care about. Your altimeter only matches it when the atmosphere matches the standard model it was calibrated to. Cold air is denser, so the pressure levels squeeze closer to the ground, and an altimeter flying through them overreads: it shows more altitude than you actually have. Hence the classic warning:

From high to low, look out below — flying into lower pressure or colder air, your true altitude is lower than indicated.

Example. Cruising at an indicated 6,000 ft over a near-sea-level station on a day 20 °C below ISA, the rule of thumb of about 4 ft of error per 1,000 ft of height per degree of ISA deviation puts you roughly 4 × 6 × 20 ≈ 480 ft lower than the needle claims. That’s why cold-temperature-restricted airports require corrected approach altitudes in winter. The same non-standard air splits indicated from true airspeed, too — the true airspeed calculator quantifies it.

Absolute altitude: height above the ground

Absolute altitude is your height AGL — above the terrain directly beneath you. Cross a 5,000 ft plateau at a true altitude of 7,500 ft MSL and your absolute altitude is 2,500 ft. It changes constantly with the terrain even when you hold a rock-steady MSL altitude.

Airliners measure it directly with a radar altimeter — the voice calling “fifty… forty… thirty” in the flare, and the basis of decision height on Category II/III approaches. Light-aircraft pilots mostly infer it from the chart, but plenty of rules are written in AGL: traffic-pattern altitude, the 500/1,000 ft minimum safe altitudes, and cloud-clearance floors.

What altitude does your altimeter actually show?

Your altimeter shows indicated altitude — nothing more. It is an aneroid pressure gauge calibrated to the standard atmosphere, so it converts outside air pressure into feet as if the day were standard. With a current local setting, indicated altitude approximates true altitude near the reporting station, but it drifts as pressure and temperature depart from standard or you fly away from the station. It never shows density altitude and it never shows height above the ground — those take a calculation and a radar altimeter, respectively.

What is the difference between pressure altitude and density altitude?

Pressure altitude is your height above the 29.92 inHg standard datum; density altitude is that same number corrected for temperature. The two are equal only when the temperature is exactly standard. On a hot day density altitude climbs above pressure altitude — about 120 ft for every degree Celsius above ISA — and on a cold day it drops below. Pressure altitude is the bookkeeping altitude you feed into charts and flight levels; density altitude is the physical altitude your airplane performs at. The two-step chain is always the same: altimeter setting gives you PA, temperature turns PA into DA.

How to find pressure and density altitude on an E6B vs. online

On a mechanical E6B, density altitude is a window job. First get pressure altitude — either read it off the altimeter with 29.92 set (then reset the local altimeter!) or use the formula above. Then, in the small pressure-altitude window, align your PA against the outside air temperature on the OAT scale, and read density altitude in the density-altitude window. Electronic E6Bs ask for the same two inputs and skip the squinting.

Online, the chain is one screen: the DensityAlt calculator takes field elevation, temperature, and altimeter setting, shows PA, ISA deviation, and DA together, and can pull live airport weather by ICAO code; the pressure altitude calculator isolates the first step. For the intuition behind all six numbers — how pressure, density, and oxygen actually fall with height — drag an aircraft up through the atmosphere explorer.

Educational only. The figures here are rules of thumb to build intuition — always fly your POH numbers and current official weather, not these.

Levan Sulakvelidze

Instrument-rated private pilot and the builder of DensityAlt. I fly out of high-and-hot fields in the western U.S., where density altitude isn’t a textbook abstraction — it’s the difference between a comfortable climb and staring down the far end of the runway. I built these tools to make the numbers obvious before the throttle goes up. LinkedIn · GitHub