Pressure altitude is the height above a theoretical reference point called the standard datum plane, where atmospheric pressure is exactly 29.92 inches of mercury (1,013.25 hPa). It tells you what your altitude “would be” if the atmosphere were behaving according to a standardized model, regardless of what the actual weather conditions are doing that day. Pilots, air traffic controllers, and engineers use it as a universal reference so everyone is working from the same numbers.
How Pressure Altitude Works
The atmosphere exerts less pressure the higher you go. At sea level on a standard day, that pressure is 29.92 inches of mercury (inHg), or 1,013.25 hectopascals (hPa). Pressure drops roughly 1 hPa for every 28 feet of altitude gained. Pressure altitude simply reverses that relationship: given a measured air pressure, it calculates the corresponding altitude in the standard atmosphere model.
When a pilot sets the altimeter to 29.92 inHg, the instrument reads pressure altitude directly. It doesn’t care whether the airplane is actually 5,000 feet above the ground or 4,800 feet. It only reports the altitude that matches the current air pressure under standard conditions. This makes it a purely pressure-based measurement, stripped of local weather effects like temperature swings or storm systems that raise or lower actual pressure at the surface.
The Standard Atmosphere Model
Pressure altitude only makes sense in the context of the International Standard Atmosphere (ISA), a simplified model of what the atmosphere “should” look like. At sea level, ISA assumes a temperature of 15°C (59°F) and a pressure of 29.92 inHg. Temperature decreases by about 2°C for every 1,000 feet of altitude gain. These values rarely match real-world conditions on any given day, but they create a fixed, predictable baseline everyone can agree on.
On a day when the sea-level pressure happens to be exactly 29.92 inHg, pressure altitude and true altitude above sea level are the same. When the local pressure is lower than standard (say, during a storm), the pressure altitude reads higher than the airplane’s true altitude. When the local pressure is higher than standard (a high-pressure system), pressure altitude reads lower. This is why pilots constantly update their altimeter settings during flight: to correct for these differences and avoid flying at the wrong actual height.
How It’s Measured in an Aircraft
An aircraft altimeter is essentially an aneroid barometer, a sealed metal capsule with corrugated sides that expands and contracts as outside air pressure changes. A system of levers and gears translates that physical movement into a reading on a dial (or, in modern cockpits, a digital display). When the altimeter’s reference pressure is set to 29.92 inHg, the instrument’s output is pressure altitude. When it’s set to the local barometric pressure instead, the output shifts to reflect altitude above sea level more accurately.
Why Pilots Switch to 29.92 at 18,000 Feet
In the United States, the FAA requires all aircraft flying at or above 18,000 feet to set their altimeters to 29.92 inHg. This transition altitude marks the boundary where everyone stops using local pressure corrections and starts flying on pressure altitude exclusively. The reason is straightforward: at high altitudes, aircraft from different regions are converging, and each region may have different local pressure readings. If one pilot is correcting for a low-pressure area and another for a high-pressure area, their altimeters could show the same number while the planes are at dangerously different actual heights. Setting everyone to 29.92 eliminates that problem. Above 18,000 feet, altitudes are expressed as “flight levels” (FL180, FL350, etc.), which are simply pressure altitude in hundreds of feet.
Calculating Pressure Altitude
There are two common ways to calculate pressure altitude, depending on how precise you need to be.
The quick method most pilots use on the ground works like this: take the difference between the current altimeter setting and 29.92 inHg, multiply by 1,000, and add that to your field elevation. If your airport sits at 2,000 feet and the current altimeter setting is 29.72 inHg, the difference is 0.20. Multiply by 1,000 to get 200, and add it to the field elevation: pressure altitude is 2,200 feet. A lower-than-standard setting means the pressure altitude is higher than the actual elevation.
The more precise engineering formula is: pressure altitude = 145,442 × [1 − (p/p₀)^0.19026], where p is the measured pressure and p₀ is standard sea-level pressure (29.92 inHg or 1,013.25 hPa). This produces the answer in feet and accounts for the fact that the pressure-altitude relationship isn’t perfectly linear. For most practical flying, the quick method is accurate enough.
Pressure Altitude vs. Density Altitude
Pressure altitude gets you partway to understanding how the atmosphere will affect aircraft performance, but it doesn’t account for temperature. That’s where density altitude comes in. Density altitude is pressure altitude corrected for non-standard temperature (and, to a lesser extent, humidity). The formula is:
Density Altitude = Pressure Altitude + [120 × (Outside Air Temperature − Standard Temperature)]
Standard temperature at any given altitude is 15°C minus 2°C for every 1,000 feet. So at a pressure altitude of 5,000 feet, the standard temperature would be 5°C. If the actual temperature is 25°C, that’s 20 degrees warmer than standard. Multiply 120 × 20 and you get 2,400 feet to add, giving a density altitude of 7,400 feet. The airplane’s engine, wings, and propeller will perform as though the aircraft is at 7,400 feet, not 5,000.
This distinction matters most during takeoff and landing. A runway at 3,000 feet elevation on a hot summer afternoon can have a density altitude of 6,000 feet or more, meaning the aircraft needs significantly more runway to get airborne. Pressure altitude is the starting point for that calculation, which is why pilots need to know it before every flight, not just when cruising at high altitude.
Other Types of Altitude
Aviation uses several altitude references, and they serve different purposes:
- Indicated altitude is what the altimeter reads when set to the local barometric pressure. It approximates your height above sea level.
- True altitude is your actual height above mean sea level, corrected for all atmospheric variables. Navigational charts reference true altitude.
- Absolute altitude is your height above the ground directly below you, measured by radar altimeters.
- Pressure altitude is your height in the standard atmosphere with the altimeter set to 29.92 inHg. It’s the common reference for high-altitude flight and performance calculations.
- Density altitude is pressure altitude adjusted for temperature, reflecting the air density the aircraft actually experiences.
Each one answers a slightly different question. Pressure altitude’s role is to provide a standardized, weather-independent number that keeps aircraft safely separated and gives pilots a reliable starting point for performance math.