Pressure altitude calculator
Pressure altitude is the altitude your aircraft thinks it is at when the altimeter subscale is set to the standard 1013.25 hPa, or 29.92 inHg. Performance charts are drawn against it, so almost every performance calculation starts here.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The answer is the height of your position above the standard pressure datum, which is the level where the atmosphere would read 1013.25 hPa. On a low pressure day that datum sits above sea level, so your pressure altitude comes out higher than your real elevation. On a high pressure day it sits below sea level and the pressure altitude comes out lower.
Pressure altitude is not how high you are. It is a way of describing the air you are in, so that a chart drawn in a test programme years ago can be applied to conditions today.
When this is useful
- Before reading any takeoff, landing or climb performance chart, since nearly all of them are indexed by pressure altitude rather than elevation.
- As the first step in working out density altitude, which needs pressure altitude before temperature can be applied.
- When flying above the transition altitude, where everyone sets 1013.25 hPa and the altimeter is showing pressure altitude directly as a flight level.
How it is calculated
The standard atmosphere defines a fixed relationship between pressure and height. Feeding the altimeter setting into that relationship gives the height of the pressure datum relative to mean sea level, and adding your field elevation gives pressure altitude.
This calculator uses the full barometric formula from the standard atmosphere rather than the familiar shortcut of 1,000 ft per inch of mercury. Near sea level the two agree closely, but the shortcut drifts as the setting moves further from standard.
Pressure altitude = field elevation + 145,442 x (1 - (QNH / 1013.25) ^ 0.190263)
Worked example
- Field elevation
- 1,200 ft
- Altimeter setting
- 1008 hPa
Pressure altitude is about 1,344 ft. The setting is 5.25 hPa below standard, which lifts the datum about 144 ft above sea level, so the aircraft sits that much higher in pressure terms than its real elevation suggests.
Check: The old rule of thumb is roughly 27 ft per hPa. 5.25 hPa multiplied by 27 gives about 142 ft, which agrees with the exact figure of 144 ft.
Limitations
- The result describes the standard atmosphere, not the real one. Actual pressure does not fall with height exactly as the model says, especially in unusual weather.
- It uses QNH, the setting that makes the altimeter read elevation on the ground. Entering QFE, which reads zero on the ground, gives a wrong answer.
- Above the transition altitude, pressure altitude and indicated altitude are the same thing, so there is nothing to calculate.
Common mistakes
- Mixing up the units of the altimeter setting. A value near 1013 is hectopascals and a value near 29.92 is inches of mercury. Use the unit selector rather than converting in your head.
- Using pressure altitude where density altitude is needed. Pressure altitude ignores temperature completely, so on a hot day it will make performance look far better than it is.
- Entering elevation above ground level instead of above mean sea level.
Sources
- Pilot's Handbook of Aeronautical Knowledge, chapter 4 , US Federal Aviation Administration
- U.S. Standard Atmosphere, 1976 (NASA-TM-X-74335) , NASA, NOAA and US Air Force
- Earth Atmosphere Model , NASA Glenn Research Center