Pressure altitude

Pressure altitude is your height above the standard-pressure datum, the 29.92" level. Roll 29.92" into the Kollsman window and the altimeter reads it directly; aircraft performance charts are drawn against it.

Three pressure words, do not mix them (FAA AWH spells them out: station pressure 8.2.3, sea-level pressure 8.2.5, pressure altitude 8.4.1.4; the FAA calls the value you dial in the altimeter setting, "QNH" being the ICAO label for it, and does not use QFE):

• Station pressure: what the barometer actually reads at the station's own elevation.
• Sea-level pressure (SLP): station pressure reduced to sea level using the day's actual temperatures, so stations at different heights compare. The altimeter setting (QNH) you dial in to make the altimeter read height above sea level is the same reduction but done with the ISA, so it is close to but not identical with reported SLP.
• Pressure altitude: the altitude you read with the altimeter set to 29.92" (the height that pressure has in the ISA).

For any setting other than 29.92", correct at 1000 ft per inch (TC AIM AIR 1.5.3): PA = indicated + (29.92 − setting) × 1000.

Unit trap: if the setting is in hPa, convert first. Standard is 29.92" = ~1013 hPa and 1 inHg = 33.86 hPa, so ~29.5 ft per hPa (do not reuse 33.86 as the feet-per-hPa factor).

A setting below 29.92 puts the standard datum below you, so your true pressure altitude is higher than the indicated altitude. In the AIM's words, a too-low subscale setting means a too-low altimeter reading, so the aircraft is actually higher than indicated (TC AIM AIR 1.5.3).

Trap: apply the correction in the right direction. A low setting (say 29.42") makes PA sit above the indicated altitude, not below.

Source note: the pressure vocabulary here (station pressure, sea-level pressure, pressure altitude, and the ~1 inHg per 1000 ft reduction) is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28), a free public-domain text; the physics is universal and matches the Canadian material (equivalent to TP 9352). The FAA uses "altimeter setting" rather than the ICAO labels QNH/QFE. The 29.92" datum, the 1000 ft per inch correction, and the too-low-setting rule stay cited to the TC AIM AIR.

Practice