Definition
An adjustment applied to indicated altitude to obtain pressure altitude when the altimeter setting is not 29.92 inches of mercury. The conversion adds or subtracts a value (in feet) based on the current altimeter setting, producing the altitude that would be shown if the altimeter were set to the standard pressure of 29.92 "Hg.
Plain English
It is the small correction you apply to your indicated altitude so you can find out what your altitude would read if the altimeter were set to the standard sea level pressure. You need this number to use most performance charts.
Context Anchor
Used before reading density altitude and aircraft performance charts, especially for takeoff, climb, and landing planning.
Derivation
"Conversion" comes from the Latin convertere, meaning to turn around or change. Here it simply means changing one altitude reading into another by applying a correction.
Why Pilots Care
Using the correct pressure altitude prevents underestimating takeoff distance, climb rate, or landing performance in non-standard atmospheric conditions.
Intuition Check
Pressure altitude conversion does not mean changing feet to meters, and it does not give height above the ground. It converts local elevation and pressure setting into an altitude based on the standard 29.92 pressure reference.
Example Sentence 1
With the altimeter set to 30.12, the pilot used the pressure altitude conversion table to subtract 165 feet from the field elevation before entering the density altitude chart.
Example Sentence 2
The pressure altitude conversion showed the airplane was actually operating 800 feet higher than the field elevation indicated.