Read first before you start considering it a duplicate, please, as the similar questions didn't answer my question either.

The usual standard atmospheres have two altitudes: the geopotential and the geometric one and I'm still confused on which is which and which one an aircraft's altimeter is showing.

  • In the U.S. 1976 Standard Atmosphere (i.e. not the ISA/ICAO but perhaps in theirs too) the geometric altitude is the true altitude above sea level while the geopotential altitude is the one that considers 1g to be at any altitude (i.e. as if gravity didn't decrease with altitude, remaining 1g all the way up). This makes sense as the pressure value is lower for the geopotential value than for the geometric value, implying that gravity pushes down more strongly because it's considered 1g, not a lower gravity, in the geopotential value.
  • The WP page however, considers the geopotential altitude to be the gravity-adjusted one. Perhaps the ISA defines the altitudes differently, and "geopotential altitude" in the ISA means what "geometric altitude" means to the U.S. SA? This link seems to support that because it says the pressure altitude is the geopotential one, and then goes on to say 262 hPa is the pressure at 33,000 ft, which in the U.S. SA is the geometric altitude value, not the geopotential one (which would be 261 hPa).
  • The U.S. Standard Atmosphere abbreviates the geometric altitude as "Z" and the geopotential altitude as "H" but I've seen the geopotential altitude be called "Z" too (or was it lowercase "z"?). The WP page calls the geopotential altitude "Z" and the link from above calls it "H" and the geometric altitude "h".

Now, to finally get rid of all confusion, I'd like to ask: when a plane's altimeter shows that it cruises at 33,000 ft (FL330), does that mean it is at 261 hPa or at 262 hPa?

  • 2
    $\begingroup$ You might consider asking on Earth Science SE. $\endgroup$ Oct 25, 2022 at 14:44
  • 3
    $\begingroup$ @MichaelHall Well technically my question is what altitude a plane's altimeter (when calibrated to 29.921 inHg) is showing. $\endgroup$
    – Giovanni
    Oct 25, 2022 at 16:29
  • $\begingroup$ Technically, yes. I wasn't suggesting that your question was off-topic, only that because of the somewhat arcane nature of the difference between the two you might get a better answer somewhere else. Certainly there is overlap... $\endgroup$ Oct 25, 2022 at 17:57

1 Answer 1


The altimeter is calibrated to the geopotential height.

At a geopotential height of 33000 ft the standard atmosphere model will give, according to ICAO doc 7488, a pressure of 262 hPa. This corresponds to a geometric height of 33052 ft AMSL.

An altimeter calibrated to the standard atmosphere and set to standard pressure (1013.25 hPa) will, when measuring a static pressure of 262hPa indicate FL330 (33000ft)

  • $\begingroup$ Very well, thank you! Obviously the U.S. Standard atmosphere confused me. Their "geometric" altitude is in fact the ISA's geopotential altitude then, which, as the WP page is stating, is the gravity-adjusted and therefore correct altitude. $\endgroup$
    – Giovanni
    Oct 25, 2022 at 16:30

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