Pressure Altitude Calculator: From Altimeter Setting or Station Pressure
Pressure altitude from field elevation and the altimeter setting (inHg or hPa) with the FAA rule (29.92 − setting) × 1,000 + elevation, the exact standard-atmosphere value, or from station pressure (NWS formula), plus ISA temperature and density altitude.
How it works
Pressure altitude = (29.92 − altimeter setting in inHg) × 1,000 + field elevation (ft) · hPa → inHg: ÷ 33.8639 · precise: station pressure P = setting × ((288 − 0.0065·h_m) ÷ 288)^5.2561, PA = 145,366.45 × [1 − (P_hPa ÷ 1013.25)^0.190284] · ISA temperature = 15 − 1.98 °C per 1,000 ft · density altitude ≈ PA + 120 × (OAT − ISA temperature)
Pressure altitude is the altitude in the standard atmosphere that matches the pressure where you are: what the altimeter would read with 29.92 inHg (1013.25 hPa) set. Performance charts for takeoff, climb and cruise are drawn against it, and above the transition altitude (18,000 ft in the US) everyone flies pressure altitudes so that vertical separation does not depend on local weather. The FAA method adds 1,000 ft for every inch of mercury the altimeter setting is below 29.92 (and subtracts when it is above), so a low-pressure day makes a 3,200 ft airport behave like one at 3,500 ft. The calculator also works out the precise standard-atmosphere value: it converts the altimeter setting back to the actual station pressure at the field with the National Weather Service formula and applies the barometric equation. If you have the station pressure itself, choose Station pressure and the NWS formula is used directly. Density altitude then corrects pressure altitude for temperature, about 120 ft for every degree Celsius above the ISA temperature for that altitude (15 °C at sea level, falling about 2 °C per 1,000 ft). Humidity raises density altitude slightly more; the dry-air NWS value is shown for comparison.
Worked example
With the defaults, a field at 3,200 ft and an altimeter setting of 29.62 inHg: pressure altitude = (29.92 − 29.62) × 1,000 + 3,200 = 3,500 ft. The precise standard-atmosphere value (station pressure 26.35 inHg) is 3,474 ft. ISA at 3,500 ft is 15 − 1.98 × 3.5 = 8.1 °C, so at 28 °C the density altitude is about 3,500 + 120 × (28 − 8.1) = 5,892 ft, and the NWS dry-air formula gives 5,750 ft: the aircraft performs as if it were almost 6,000 ft up.
Sources: National Weather Service: pressure altitude formula (from station pressure) · National Weather Service: station pressure from altimeter setting and elevation · National Weather Service: density altitude formula · FAA Aeronautical Information Manual, chapter 7-2: barometric altimeter errors and setting procedures · ThrustFlight: pressure and density altitude worked example (Aspen, 7,837 ft, 30.05 inHg)
Frequently asked questions
How do I calculate pressure altitude?
Subtract the altimeter setting from 29.92, multiply by 1,000 and add the field elevation. At 7,837 ft with 30.05 inHg: (29.92 − 30.05) × 1,000 + 7,837 = 7,707 ft. A high setting gives a pressure altitude below the field elevation; a low setting gives one above it. You can also set 29.92 in the altimeter on the ground and read it directly.
How do I do it with QNH in hPa?
Convert hPa to inHg by dividing by 33.8639 and use the same rule, or use the near-sea-level rule of about 27–30 ft per hPa below 1013.25. With 1003 hPa (29.62 inHg) at a 3,200 ft field the pressure altitude is about 3,500 ft, or 3,474 ft with the precise formula.
What is the difference between QNH, QFE and QNE?
QNH is the setting that makes the altimeter read field elevation on the ground; QFE makes it read zero. QNE is the standard 29.92 inHg (1013.25 hPa) setting, which makes the altimeter show pressure altitude.
What is the difference between pressure altitude and density altitude?
Pressure altitude corrects only for non-standard pressure. Density altitude also corrects for temperature (and humidity): on a hot day the air is thinner, so engines, propellers and wings perform as if the aircraft were higher. Use about 120 ft per °C above the ISA temperature.
Why does the precise value differ from the 1,000 ft rule?
Pressure falls exponentially with height, so one inch of mercury equals about 920 ft near sea level and more higher up. The 1,000 ft rule is the standard pilot method and is close enough for performance charts; the precise value uses the standard-atmosphere equations used by the National Weather Service.
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