Formula
Pressure altitude is the height in the International Standard Atmosphere where the pressure equals the pressure you have. With 29.92 inHg set in the Kollsman window, it is the altitude the altimeter reads. The FAA rule moves the field elevation by 1,000 feet for each inch the reported altimeter setting differs from 29.92. A setting above 29.92 lowers pressure altitude.
PA = elevation + (29.92 − setting) × 1,000
PA ≈ elevation + (1,013.25 − QNH) × 30
PA = 145,366.45 × [1 − (P / 1,013.25)^0.190284]
T = 15 − 1.98 × PA / 1,000
The hectopascal line is a rule of thumb, about 30 feet per hPa, and it is labelled approximate. The station-pressure line is the NOAA form of the ICAO troposphere. One inch of mercury is taken as 33.8639 hPa when you enter station pressure in inches. ISA temperature here uses 1.98 °C per 1,000 feet. The density altitude page uses a 2 °C lapse in its own rule of thumb. They are neighbouring approximations, not two atmospheres.
Worked examples
5,000 ft, altimeter 30.12 inHg. 4,800 ft. ISA temperature 5.5 °C. 1,200 ft, 29.70 inHg. 1,420 ft. Sea level, 29.92. 0 ft.
500 ft, QNH 1003 hPa. 807.5 ft by the 30-foot rule.
Station pressure 843.07 hPa. About 4,998 ft. 1,013.25 hPa. 0 ft. 1,003 hPa. About 281 ft.
Reference
The 1,000-foot rule is linear. It is the number pilots use in the airplane. The station-pressure formula is the one to use when you already have the pressure at the field, not the altimeter setting reduced to sea level.
Questions
How do you calculate pressure altitude?
Add the field elevation to one thousand feet for every inch of mercury the altimeter setting sits below 29.92, and subtract when the setting is above 29.92. At 5,000 feet with 30.12 in the window, that is 4,800 feet.
What is the pressure altitude formula?
The FAA rule is elevation plus (29.92 minus the altimeter setting) times 1,000. From station pressure, the NOAA form of the ICAO troposphere is 145,366.45 times one minus (pressure divided by 1,013.25) to the power 0.190284.
How is pressure altitude different from density altitude?
Pressure altitude is the standard-atmosphere height of the pressure you have. Density altitude is that height adjusted for temperature and humidity, because hot air is less dense. Performance charts that ask for density altitude want the second number. This page stops at pressure altitude.
How do you find pressure altitude in the airplane?
Set 29.92 inHg in the Kollsman window. The altitude the instrument then shows is pressure altitude. Above 18,000 feet in the United States, flight levels are flown with that setting.
What is pressure altitude at sea level?
On a standard day the altimeter setting is 29.92 inHg and the pressure altitude is the field elevation. At sea level that is 0 feet. A higher setting makes pressure altitude lower than the field.
How many feet per inch of mercury?
The FAA rule uses 1,000 feet per inch of mercury. One hectopascal is about 30 feet, and that second figure is labelled approximate on this page.
What is pressure altitude used for?
It is the entry some aircraft performance charts want before temperature is applied, and it is the basis of flight levels. Use the POH or AFM and official weather for a flight.
Sources
- FAA Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25. Pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000. Flight levels in the United States are flown on 29.92 above 18,000 feet.
- NOAA National Weather Service pressure-altitude formula, and ICAO Doc 7488, the International Standard Atmosphere. PA (ft) = 145,366.45 × [1 − (P / 1013.25)^0.190284], with P in hectopascals. ISA temperature on this page is 15 − 1.98 × PA/1,000.
- The 30 feet per hectopascal line is the common rule of thumb. It is marked approximate. One inch of mercury is converted at 33.8639 hPa when station pressure is entered in inches.
- The density altitude calculator on this site uses a nearby ISA inversion (scale 145,442.16 ft and exponent 5.25588) for moist density. This page does not replace that calculation.