Definition
The internationally agreed standard sea-level atmospheric pressure value, equivalent to 29.92 inches of mercury (inHg) or 1013.2 hectopascals (hPa). It is the baseline pressure used in the International Standard Atmosphere (ISA) and is the value pilots set on the altimeter when flying at or above the transition altitude to display pressure altitude (flight level).
Plain English
It is the standard air pressure at sea level that the world has agreed to use as a reference. When you set this number on your altimeter, every aircraft is measuring height from the same imaginary starting line in the sky.
Context Anchor
Seen in atmospheric pressure, standard atmosphere, altimeter setting, and high-altitude operations discussions.
Derivation
A millibar is one-thousandth of a 'bar,' a pressure unit named from the Greek 'baros' meaning 'weight.' The number 1013.2 was chosen because it represents the average weight of the atmosphere pressing down on sea level — not a perfect or fixed real-world value, but an agreed standard.
Why Pilots Care
This fixed value lets pilots and manufacturers calculate takeoff distance, climb rate, and true altitude without local pressure variations.
Analogy
Think of 1013.2 millibars like a marked reference line on a measuring stick. The day’s actual pressure may be above or below it, but everyone can compare from the same starting point.
Grounding Statement
On a standard day at sea level, the air pressing down on the surface is treated as 1013.2 millibars.
Intuition Check
Do not read 1013.2 millibars as the pressure you will always have at sea level. It is a standard reference value; real air pressure changes with weather and location.
Example Sentence 1
Climbing through the transition altitude, the pilot set 1013.2 on the altimeter and reported reaching the assigned flight level.
Example Sentence 2
Performance tables in the aircraft manual assume 1013.2 millibars at sea level to give consistent numbers for takeoff and landing distances.