What is pressure altitude based on?

Pressure altitude is determined based on standard air pressure at sea level, which is defined as 29.92 inches of mercury (inHg) or 1013.25 millibars (hPa). This standard reference point is used to calibrate altimeters, allowing them to provide altitude readings consistent across various atmospheric conditions. Pilots adjust their altimeters to this standard pressure value to ensure that the altitude readings reflect the actual elevation above mean sea level (MSL), rather than being influenced by local weather patterns or variations in barometric pressure. When an altimeter is set to this standard, the reading is referred to as pressure altitude and is crucial for navigation and maintaining safe flight levels, particularly in areas where terrain and airspace are congested.