Pressure altitude is the altitude your aircraft “thinks” it is flying at when the altimeter is set to 29.92 inches of mercury (1013.25 hPa). You find it by setting that standard reference in the Kollsman window and reading the altimeter, or by calculating it from field elevation and the local altimeter setting. It matters for aircraft performance, and it is the reference used above a certain altitude in flight.
What Is Pressure Altitude in Simple Terms?
Pressure altitude is a standardized way to describe altitude based on atmospheric pressure rather than actual height above sea level. It assumes a theoretical atmosphere that matches the International Standard Atmosphere — a model where sea level pressure is 29.92 inHg and temperature decreases with height at a predictable rate.
Real weather never matches that model exactly. High-pressure days push pressure up. Low-pressure days pull it down. Storms, temperature swings, and even the time of day shift the numbers. Pressure altitude corrects for pressure changes but not temperature, which is why it is a reference rather than a true height.
Pilots use it because aircraft performance — takeoff distance, climb rate, engine output — depends on air density. Air density depends on pressure, temperature, and humidity. Pressure altitude captures the pressure part in one clean number.
How To Find Pressure Altitude Using Your Altimeter
The fastest method is to turn the altimeter setting knob until 29.92 appears in the Kollsman window. Whatever the altimeter needle reads at that setting is your pressure altitude.
This works in any aircraft with a sensitive altimeter. You do not need a chart, a calculator, or an internet connection. The instrument does the work for you.
One practical note: do this on the ground before takeoff and note the value. Then reset the altimeter to the current local setting for normal flight. Leaving 29.92 in the window during a local flight will give you readings that do not match actual field elevations.
How To Calculate Pressure Altitude From Field Elevation
If you know your field elevation and the current altimeter setting, you can calculate pressure altitude without touching the aircraft.
The standard formula is:
Pressure Altitude = (29.92 − Altimeter Setting) × 1,000 + Field Elevation
The altimeter setting must be in inches of mercury. Field elevation is in feet. The result is in feet.
This formula comes from the fact that pressure changes roughly 1 inch of mercury per 1,000 feet of altitude in the lower atmosphere. It is an approximation, but it is the one used in flight planning and it is accurate enough for performance calculations.
Worked example: Field elevation is 1,200 feet. Altimeter setting is 30.12 inHg.
- 29.92 − 30.12 = −0.20
- −0.20 × 1,000 = −200 feet
- −200 + 1,200 = 1,000 feet pressure altitude
When the altimeter setting is above 29.92, pressure altitude is lower than field elevation. When it is below 29.92, pressure altitude is higher.
Why Pressure Altitude Drops When the Barometer Rises
This trips up a lot of people. High pressure means more air stacked above you, which pushes the pressure level corresponding to 29.92 higher into the atmosphere. Your field sits below that level, so its pressure altitude reads lower.
Think of it as a seesaw. The altimeter setting and pressure altitude move in opposite directions.
- Altimeter setting above 29.92 → pressure altitude below field elevation
- Altimeter setting below 29.92 → pressure altitude above field elevation
- Altimeter setting exactly 29.92 → pressure altitude equals field elevation
This relationship is the single most common source of errors in manual pressure altitude calculations. Getting the sign wrong can throw off your performance numbers by hundreds of feet.
Pressure Altitude vs Density Altitude: What Is the Difference?
Pressure altitude and density altitude are related but not the same. Pressure altitude accounts only for pressure. Density altitude accounts for pressure, temperature, and to a smaller degree humidity.
On a hot day at a high-elevation airport, pressure altitude might read 5,000 feet while density altitude reads 8,000 feet or higher. The aircraft performs as if it were at 8,000 feet, not 5,000.
This is why pressure altitude alone is not enough for takeoff and climb planning on warm days. Density altitude is the number that reflects actual performance.
| Factor | Pressure Altitude | Density Altitude |
|---|---|---|
| Pressure | Yes | Yes |
| Temperature | No | Yes |
| Humidity | No | Small effect |
| Used for | Flight levels, altimeter reference | Aircraft performance planning |
Pressure altitude is the input you use to find density altitude, not a replacement for it. Most performance charts require density altitude, not pressure altitude.
Where Pressure Altitude Is Used in Flight
At and above 18,000 feet MSL in the United States, all aircraft set their altimeters to 29.92. This creates a common reference so that aircraft flying in opposite directions at the same indicated altitude are actually at different heights. The altitude flown at 29.92 is called a flight level — flight level 180 means 18,000 feet pressure altitude.
Below that transition altitude, aircraft use local altimeter settings so that indicated altitude roughly matches height above sea level near the ground. This matters for terrain clearance and for traffic pattern work.
Pressure altitude also shows up in aircraft performance charts, engine power settings, and some weather products. When a chart says “pressure altitude,” it means the value at 29.92, not your indicated altitude on a local setting.
A detail worth knowing: the transition altitude is not 18,000 feet everywhere. Many countries use different values, and some use a transition level that changes with local pressure. The 18,000-foot figure is a US convention, not a worldwide standard.
Common Mistakes When Finding Pressure Altitude
The math is simple, which makes the errors easy to miss.
- Wrong sign. Subtracting the altimeter setting from 29.92 in the wrong order flips the result. Always do 29.92 minus the setting.
- Using millibars. The formula needs inches of mercury. If your source gives hPa, convert first, or use a formula built for that unit.
- Confusing it with density altitude. They are not interchangeable, especially on hot days.
- Forgetting temperature. Pressure altitude ignores temperature entirely. Performance planning cannot.
- Reading the wrong field elevation. Use the published airport elevation, not an estimate.
Rounding is another quiet source of error. Small rounding in the altimeter setting can shift the result by tens of feet, which is usually fine for planning but not for precise work.
Does Pressure Altitude Change During a Flight?
Yes. As you climb, pressure drops and pressure altitude increases. As you fly into a different pressure system, the pressure altitude at a given true altitude changes too.
This is why altimeters need periodic resetting below the transition altitude. A flight from a high-pressure area into a low-pressure area without a reset can put you lower than your altimeter suggests.
The old memory aid is “high to low, look out below.” Flying from high pressure to low pressure without adjusting the altimeter means your actual altitude is lower than indicated. The reverse is also true.
Above the transition altitude, everyone uses 29.92, so the reference is consistent across aircraft. The tradeoff is that your indicated altitude no longer matches true altitude, which is why terrain clearance is handled through assigned flight levels and minimum safe altitudes rather than by reading the altimeter directly.
Tools That Give You Pressure Altitude
You do not have to do the math by hand. Several tools produce pressure altitude directly.
- Your aircraft altimeter, set to 29.92
- An electronic flight bag app with a pressure altitude function
- Aviation weather products that report pressure altitude for a given station
- A manual calculation using the formula above
Electronic tools are convenient but not infallible. They depend on accurate input. If the altimeter setting you enter is wrong, the output is wrong. Checking the result against a quick manual calculation is a reasonable habit, especially before a high-density-altitude departure.
Frequently Asked Questions
How do I find pressure altitude quickly?
Set your altimeter to 29.92 and read the indicated altitude. That reading is your pressure altitude.
What is the formula for pressure altitude?
Pressure altitude equals (29.92 minus the altimeter setting) times 1,000, plus field elevation. The altimeter setting must be in inches of mercury.
Is pressure altitude the same as density altitude?
No. Pressure altitude accounts only for atmospheric pressure, while density altitude also factors in temperature and humidity.
Why do pilots use 29.92 for pressure altitude?
It provides a common reference so all aircraft above the transition altitude use the same altimeter setting. In the United States, that transition altitude is 18,000 feet MSL.
