Pressure altitude is your height above the standard pressure level of 29.92 in Hg, while density altitude is that same pressure altitude corrected for nonstandard temperature (and humidity) to describe how the air actually performs. You always calculate pressure altitude first, then feed it into density altitude.
Part of our Aircraft Performance guide.
The four altitudes at a glance
There are four altitudes every private pilot has to keep straight: indicated, true, pressure, and density. They sound interchangeable, but each answers a different question, and mixing them up is a classic checkride trap.
- Indicated altitude is what your altimeter reads with the current local setting in the window. It is what you fly by day to day.
- True altitude is your actual height above mean sea level (MSL), which is what terrain and obstacle numbers on the sectional are measured against.
- Pressure altitude is your height above the standard 29.92 in Hg reference plane. It is the number performance charts and high-altitude flight levels are built on.
- Density altitude is pressure altitude corrected for temperature and humidity. It is the number the airplane actually flies at.
This page owns pressure altitude: what it is, why the magic number is 29.92, how to read it, and how it hands off to density altitude. For the full temperature correction and takeoff math, jump to the density altitude page.
The four types of altitude compared
Here is the whole family in one view. Notice that pressure and density altitude are computed values, not something you simply read out the window.
| Altitude | Definition | How you get it | What it is used for |
|---|---|---|---|
| Indicated | What the altimeter shows with the current altimeter setting in the window | Read directly off the altimeter once you set the local setting from ATIS, tower, or a METAR | Flying assigned altitudes, staying clear of terrain and traffic below 18,000 ft |
| True | Actual height above mean sea level (MSL) | Indicated altitude corrected for nonstandard temperature; charts and GPS report it | Terrain and obstacle clearance, since chart elevations are MSL |
| Pressure | Height above the standard datum plane (29.92 in Hg) | Set 29.92 in the window, or use field elevation + (29.92 minus setting) x 1,000 | Entering performance charts, flying flight levels, computing density altitude |
| Density | Pressure altitude corrected for nonstandard temperature and humidity | Correct pressure altitude for OAT on an E6B, a CX-3, or a POH chart | Predicting real lift, engine power, climb rate, and true airspeed |
What is pressure altitude, and why 29.92?
Pressure altitude is your vertical distance above the standard datum plane, the imaginary level where the atmospheric pressure equals 29.92 in Hg. Because air pressure drops as you climb (by roughly 1 in Hg per 1,000 ft near the surface), a single pressure value marks a single height in a standardized atmosphere. Pin the altimeter to that pressure and it reports how far above the datum plane you are.
The number 29.92 in Hg (1013.25 hPa) is not arbitrary. It is the sea level pressure of the International Standard Atmosphere (ISA), the reference model aviation is built on. The ISA also fixes sea level temperature at 15 C, a temperature lapse of about 2 C per 1,000 ft, and a pressure lapse of about 1 in Hg per 1,000 ft. By agreeing on one baseline, every altimeter, every POH chart, and every airplane can speak the same language regardless of the weather that day.
That standardization is exactly why pressure altitude exists. Local barometric pressure is constantly changing, so your true height for a given indicated reading drifts with the weather. Pressure altitude strips the weather back out by referencing everything to 29.92, giving engineers and pilots a fixed yardstick. Your altimeter reads pressure altitude and indicated altitude through the same instrument, tied to the pitot-static system; the only difference is which setting is in the Kollsman window.
How to set 29.92 and read pressure altitude
The fastest way to get pressure altitude is to let the altimeter do the work. On the ground or in cruise, follow these four steps:
- Note your indicated altitude and the current altimeter setting while parked at a known field elevation or level in cruise.
- Turn the altimeter’s Kollsman knob until the setting window reads 29.92 in Hg.
- Read the altitude the needles now show. That value is your pressure altitude.
- Turn the knob back to the current altimeter setting so the altimeter again shows your correct indicated altitude before you fly.
Do not forget step four. Leaving 29.92 in the window on a low-pressure day will make the altimeter read high and put you lower than you think, which is a real terrain hazard below 18,000 ft.
How to calculate pressure altitude with no altimeter
You do not need the airplane to find pressure altitude. When you are planning at the kitchen table or the altimeter is not handy, use the formula:
Pressure altitude = field elevation + (29.92 minus current altimeter setting) x 1,000
The logic is the 1 in Hg per 1,000 ft pressure lapse. Every hundredth of an inch the local setting sits below 29.92 raises your pressure altitude, and every hundredth above it lowers pressure altitude.
Worked example: your field elevation is 1,000 ft and the reported altimeter setting is 29.42 in Hg.
- 29.92 minus 29.42 = 0.50
- 0.50 x 1,000 = 500 ft
- 1,000 + 500 = 1,500 ft pressure altitude
Because the day is lower than standard pressure, your pressure altitude (1,500 ft) sits above your field elevation (1,000 ft). Grab the altimeter setting straight off the nearest report; see reading METARs and TAFs for where that value lives in the observation.
How pressure altitude feeds density altitude
Pressure altitude is the first half of the density altitude calculation; density altitude is pressure altitude corrected for temperature above or below standard. The rule of thumb is to add about 120 ft to pressure altitude for every 1 C the outside air temperature (OAT) sits above the ISA temperature for that pressure altitude, where ISA temp = 15 minus 2 x (pressure altitude / 1,000).
Continuing the example above, at a pressure altitude of 1,500 ft the standard temperature is about 15 minus 3, or 12 C. If the OAT is 27 C, you are 15 C above standard, so you add 15 x 120 = 1,800 ft, giving a density altitude of roughly 3,300 ft. The airplane will fly as though the field were 3,300 ft up, not 1,000. High, hot, and humid all push density altitude up.
That is the short version. For the full E6B and chart method, humidity effects, and worked takeoff numbers, go to the density altitude page.
Why it matters for performance, TAS, and safety
Pressure altitude is the value your POH performance charts are indexed to, so getting it right is the entry point to every real number you plan with. Takeoff, climb, cruise, and landing tables all ask for pressure altitude (plus temperature, weight, and wind) before they will give you a distance or a fuel burn. Feed in the wrong altitude and every number downstream is wrong too. That flows straight into your flight planning and your weight and balance decisions.
Pressure altitude also drives true airspeed. As density altitude climbs, your true airspeed (TAS) runs faster than your indicated airspeed by roughly 2 percent per 1,000 ft, which matters for groundspeed, ETAs, and fuel. And at and above 18,000 ft MSL, every aircraft sets 29.92 and flies by flight levels, so pressure altitude literally becomes the altitude everyone shares.
The safety payoff is blunt: high density altitude reduces wing lift, engine power, and propeller thrust, so takeoff and landing distances stretch and climb rate shrinks. Pilots who skip the pressure-to-density altitude step are the ones who run out of runway or fail to out-climb terrain on a hot day. Do the pressure altitude first, correct it for temperature, and you will know before you push the throttle whether the airplane can actually do what you are asking.
What you'll need
Convert any altimeter setting into pressure and density altitude with the same flight computers and reference every student pilot relies on, all from PilotMall.com.
The manual E6B computes density altitude, true airspeed, and wind on its slide-rule and wind sides and is permitted on both the FAA knowledge and practical tests. The CX-3 is the electronic upgrade that runs the same math faster (and unlike the paper E6B, it also handles weight and balance). New to the tools? Start with our E6B buyer’s guide. Pressure altitude, the standard atmosphere, and the charts they drive are all covered in PHAK.
Frequently asked questions
What is the difference between pressure altitude and density altitude?
Pressure altitude is your height above the standard pressure level of 29.92 in Hg, while density altitude is that pressure altitude corrected for nonstandard temperature and humidity. Pressure altitude is a fixed reference; density altitude tells you how the airplane will actually perform in the air you have today.
How do you find pressure altitude?
Set your altimeter to 29.92 in Hg and read the altitude the needles show, that is your pressure altitude. If you cannot adjust the altimeter, calculate it as field elevation plus (29.92 minus the current altimeter setting) multiplied by 1,000.
Is pressure altitude the same as indicated altitude?
No. Indicated altitude is what the altimeter shows with the current local altimeter setting in the window, while pressure altitude is what it shows with 29.92 set. They match only when the local altimeter setting happens to be exactly 29.92.
Why is 29.92 inches of mercury the standard?
29.92 in Hg (1013.25 hPa) is the sea level pressure of the International Standard Atmosphere, the reference model the FAA uses so every aircraft and performance chart shares one baseline. Pairing it with 15 degrees C at sea level gives pilots a common yardstick for altitude and performance.
When do pilots actually use pressure altitude?
Pilots use pressure altitude to enter takeoff, climb, and landing performance charts, to compute density altitude and true airspeed, and to fly by flight levels at and above 18,000 ft where everyone sets 29.92. It is the standardized altitude that makes performance planning consistent.