Definition
An altimeter setting of 1013 hectopascals (hPa), which is the standard atmospheric pressure at sea level used internationally to establish flight levels. When an altimeter is set to 1013 hPa, it displays pressure altitude rather than altitude above mean sea level, allowing all aircraft operating above the transition altitude to use a common reference and maintain consistent vertical separation regardless of local pressure variations.
Plain English
When pilots set their altimeter to the value 1013, they are using the worldwide standard pressure setting. Above a certain altitude, all aircraft switch to this same setting so that everyone is measuring height from the same baseline, which keeps aircraft properly separated from each other.
Context Anchor
Seen in altimeter-setting procedures, flight level discussions, and international aviation material that uses hectopascals instead of inches of mercury.
Derivation
1013 refers to 1013.25 hectopascals (hPa), also expressed as millibars, defined as the standard atmospheric pressure at mean sea level under the International Standard Atmosphere (ISA). The hectopascal is the metric unit of pressure used in most of the world outside the United States.
Why Pilots Care
Using this single reference pressure above the transition altitude guarantees that every aircraft in the same airspace maintains vertical separation even when local sea-level pressure varies.
Analogy
It is like setting several rulers to start from the same zero mark before comparing measurements. The actual ground below each airplane may be different, but the altitude readings are being measured from the same agreed reference.
Intuition Check
Do not read this as meaning the outside air pressure around the airplane is exactly 1013. It means the altimeter is adjusted to use 1013 as its reference setting.
Example Sentence 1
Climbing through the transition altitude, the pilot set the altimeter to 1013 and reported reaching Flight Level 180.
Example Sentence 2
With the altimeter set to a pressure of 1013, the pilot reported level at flight level 350 and maintained that pressure surface for the remainder of the cruise.