Pressure Altitude & Altimeter Setting
Pressure altitude from field elevation and altimeter setting, exact through the ICAO atmosphere with the 30 ft per hPa rule beside it; inHg ↔ hPa and QFE.
The pressure altitude for a field and QNH, the correction that was added to the elevation, the rule-of-thumb figure and how far it drifts, the station pressure in both units, and the ISA temperature at that altitude.
Example: A 5,000 ft field with the altimeter at 29.50 inHg (999.0 hPa): the exact correction is 396 ft, so pressure altitude 5,396 ft, where the 1,000 ft per inch rule says 5,420.
Standard height
of the QNH.
What pressure altitude is, how the exact figure follows from the ISA pressure law, where the rule of thumb comes from and when it drifts, and what the station pressure means.
Definition
Pressure altitude is the height in the standard atmosphere at which the pressure equals the actual pressure — what the altimeter shows with 29.92 inHg (1013.25 hPa) set. With the local QNH set the altimeter reads the field elevation, so pressure altitude is the elevation plus the ISA height of the QNH pressure: h = (T₀ ÷ L) × [1 − (QNH ÷ 1013.25)^(1/5.2559)] from the ICAO law, positive when the QNH is below standard.
The rule of thumb
Near sea level the standard pressure falls about 1 hPa every 27 ft, or one inch of mercury every thousand feet, so (1013.25 − QNH) × 30 ft and (29.92 − QNH) × 1,000 ft are the working rules. They are linear; the atmosphere is not, so the drift grows with the distance from standard — the page shows the difference against the exact figure.
Station pressure
The pressure actually at the field (QFE) is the ISA pressure at the pressure altitude; the page gives it in hPa and inHg, and the ISA temperature at that altitude, which the density-altitude page uses as its baseline.
What the number does not settle
Pressure altitude says nothing about density: for take-off and climb performance go on to Density Altitude with the temperature. Altimeter error and the transition altitude rules of the airspace are not modelled. A planning aid, not a substitute for the pilot’s operating handbook, the aircraft’s limitations or the rules that apply to the flight. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.
SOURCES
- ICAO Doc 7488, Manual of the ICAO Standard Atmosphere (constants and layer laws)
Last reviewed 23 September 2026. How results are checked: How we verify.