Definition
The international standard sea-level atmospheric pressure value used as the reference setting for altimeters when flying at flight levels. 1013.2 hectopascals (hPa) and 1013.2 millibars (mb) are numerically identical units of pressure; the hectopascal is the modern SI unit and has replaced the millibar in most aviation publications, though the two terms are used interchangeably. This value corresponds to 29.92 inches of mercury (inHg).
Plain English
It is the standard pressure setting pilots dial into the altimeter when flying high enough to use flight levels. Setting this value means every aircraft above the transition altitude is measuring height from the same agreed reference, so they can be safely separated even if the actual local pressure is different.
Context Anchor
Seen in altitude, altimeter-setting, and instrument procedure discussions, especially when comparing U.S. pressure settings with international pressure units.
Derivation
Hectopascal comes from 'hecto-' (Greek for one hundred) and 'pascal' (named after Blaise Pascal, the French scientist who studied pressure). One hectopascal equals one hundred pascals. Millibar comes from 'milli-' (Latin for one thousandth) and 'bar' (Greek 'baros' meaning weight). One hectopascal and one millibar are exactly equal -- the units were renamed but the value did not change.
Why Pilots Care
Using this single reference value prevents altitude conflicts between aircraft and ensures consistent vertical separation when local pressure settings are no longer applied.
Analogy
Think of it like agreeing on the zero mark of a ruler before measuring. The airplane can only show altitude consistently if everyone is using the same pressure reference.
Grounding Statement
1013.2 hPa or mb is the standard pressure reference used to make altitude readings consistent when standard pressure is required.
Intuition Check
Do not read “hectopascals or millibars” as two different pressures. In this aviation use, 1013.2 hectopascals and 1013.2 millibars are the same pressure value.
Example Sentence 1
Climbing through the transition altitude, the pilot set the altimeter to 1013.2 hPa and began reporting flight levels.
Example Sentence 2
In regions using millibars the crew set 1013.2 millibars on the standby altimeter before entering the high-level airway.