What pressure altitude is
Pressure altitude is the altitude in the standard atmosphere at which the pressure equals the pressure at your location. Operationally, it is what a sensitive altimeter reads when 29.92 inHg (1013.25 hPa) is set in the Kollsman window. Performance charts, true-airspeed calculations, and density altitude all start from it.
The altimeter is calibrated to the ICAO ISA troposphere. Station pressure is obtained from QNH and elevation by the same hydrostatic relation, and pressure altitude is the inverse of that relation — the same constants used on this site’s density altitude calculator (T0 = 288.15 K, L = 0.0065 K/m, g0 = 9.80665 m/s², R = 287.05287 J/(kg·K)).
The 1,000-foot rule of thumb
The memory item is PA ≈ elevation + 1,000 ft × (29.92 − QNHinHg). It is close, and it is what most pilots use on the ramp, but the ISA scale height is not exactly 1,000 ft per inch of mercury, so the exact altimeter calibration differs by a few tens of feet at a 1.00 inHg departure from standard. This page shows both.
QNH, QNE, and QFE
QNH is the altimeter setting that makes the instrument read field elevation on the ground. QNE is the standard setting 29.92 inHg / 1013.25 hPa, used for flight levels. QFE is the setting that makes the altimeter read zero at the field (it is the station pressure expressed as an altimeter setting). This page reports all three.
Sources
ICAO Doc 7488; FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 8 (altimetry, pressure altitude); AIM altimeter-setting procedures; U.S. Standard Atmosphere 1976. For the full ISA column, see the standard atmosphere calculator.
Frequently asked questions
If I set 29.92 on the ground at a 5,000 ft field, is my pressure altitude 5,000 ft?
Yes, when the official QNH is itself 29.92 inHg (standard). If QNH is lower than 29.92, pressure altitude is higher than field elevation, and the rule of thumb is 1,000 ft of PA for each inch of mercury the setting is below 29.92.
Is 29.92 inHg exactly 1013.25 hPa?
1013.25 hPa is 29.92126 inHg using the conversion 1 inHg = 33.8638867 hPa. A typed 29.92 on this page is treated as that conventional standard, not as 29.92000, so a standard-day sea-level or field case comes out at the elevation you entered, not a foot high.
How is this different from density altitude?
Pressure altitude depends only on pressure. Density altitude also needs temperature (and, optionally, humidity). On a hot day, density altitude is higher than pressure altitude. Use the density-altitude calculator for performance estimates.