Pressure altitude is the altitude read from an altimeter when the instrument is set to the standard sea-level pressure of 29.92 inHg (inches of mercury), also written as 1013.25 hPa. It measures how high the aircraft is relative to the standard pressure datum, not actual sea level.
How It Works#
Every altimeter works by sensing the surrounding air pressure. As altitude increases, air pressure drops. The altimeter converts that pressure drop into a height reading.
When you set the altimeter's Kollsman window to 29.92 inHg, you remove local pressure variations from the equation. The instrument now reads pressure altitude directly. This gives every aircraft a common reference point, regardless of local weather.
Contrast this with indicated altitude, which uses the local altimeter setting (called the QNH) to correct for regional pressure. Pressure altitude ignores that correction. The difference between the two can be several hundred feet on a day when local pressure differs significantly from standard.
Temperature also matters. Standard atmosphere assumes a sea-level temperature of 15°C and a lapse rate of 2°C per 1,000 feet. On a hot day, the air is less dense than standard, and true altitude will be higher than pressure altitude suggests. This is the foundation of density altitude calculations.
Example in Aviation#
A pilot departs a field at 5,000 feet MSL (mean sea level) on a hot summer day. The local altimeter setting is 29.72 inHg. When the pilot sets the Kollsman window to 29.92 inHg, the altimeter reads approximately 5,200 feet. That 5,200-foot reading is the pressure altitude.
The pilot needs this figure to calculate density altitude and confirm the aircraft's performance data. Engine power charts, climb rates, and takeoff distances in the Pilot's Operating Handbook all reference pressure altitude as a starting point.
Why It Matters#
Above 18,000 feet MSL in the United States, all aircraft must set their altimeters to 29.92 inHg per 14 CFR §91.121. This airspace, called Class A, uses pressure altitude as a universal reference so that all aircraft share the same vertical separation standard. Controllers assign flight levels, which are pressure altitudes expressed in hundreds of feet, such as FL350 for 35,000 feet.
Below 18,000 feet, pressure altitude still drives performance calculations. Pilots who skip this step risk underestimating takeoff roll, overestimating climb rate, and misjudging service ceiling. On a hot, high-elevation day, those errors can be dangerous.
Key Takeaways#
- Pressure altitude is the altimeter reading with the subscale set to 29.92 inHg (1013.25 hPa).
- It provides a universal reference independent of local weather and pressure.
- All flight levels use pressure altitude above 18,000 feet MSL in the US.
- Aircraft performance charts reference pressure altitude, not indicated altitude.
- Pressure altitude is the starting point for calculating density altitude.