04 · Performance · Pressure altitude
Pressure altitude .
The altitude you would read if you set 29.92 inches of mercury. The reference your performance charts and the density-altitude calculation both expect.
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Section 01
Compute
Section 02
Pressure altitude, explained
An altimeter is a barometer with a face calibrated in feet rather than inches of mercury. Set 29.92 in Hg in the Kollsman window and it reads the altitude that pressure thinks you're at, irrespective of the actual local pressure. That number is pressure altitude.
Why care? Because every performance chart in your POH — takeoff distance, climb rate, cruise
true airspeed — is tabulated against pressure altitude, not field elevation. The formula is
the FAA-standard approximation:
PA = field-elevation + (29.92 − altimeter) × 1,000. One inch of pressure mismatch
lifts the pressure altitude by 1,000 ft above the field on a low-pressure day (and drops it
1,000 ft below on a high-pressure day).
For international operations, altimeter settings are reported in hectopascals (millibars). The
conversion is 1 in Hg ≈ 33.8639 hPa; standard pressure is 1013.25 hPa. The
calculator above shows both for convenience.
Section 03
A worked example
KDEN with an altimeter setting of 30.10 in Hg — a typical high-pressure day. Higher-than-standard pressure pushes the air column down, so the same field elevation reads as a lower pressure altitude — 180 ft below the field. This is the input you'd carry into the density-altitude calculator and into every performance chart in the POH.
Inputs
- Field elevation
- 5,433ft
- Altimeter
- 30.10in Hg
Output
- Correction
- −180ft
- Pressure altitude
- 5,253ft
Section 04
Common pitfalls
- 01The 1,000-ft-per-inch rule is an approximation. Near sea level the actual rate is closer to 1,054 ft per inch — accurate enough for performance work but don't use this for high-precision altimeter calibration.
- 02Field elevation is published in the FAA chart supplement / A/FD. It is not the runway-end elevation, which can vary by tens of feet across a long runway.
- 03On a high-pressure day, pressure altitude is below field elevation — a negative-correction case. The formula handles it (the sign just flips); don't substitute 0 ft if your math says −200 ft.
- 04Hectopascals are sometimes called millibars on European charts. They're the same unit; 1 hPa = 1 mb.
- 05Pressure altitude is not the same as indicated altitude (what your altimeter reads) or true altitude (your actual height above sea level). All three differ on a non-standard day.
Download
Free on the App Store. iOS 18 and up.
The iOS app derives pressure altitude live from each watched station's METAR — visible on the station detail screen and used as the input to the density-altitude and best-runway calculations.