What is the transition altitude?

The transition altitude is the altitude at which pilots switch from using altimeters set to local barometric pressure to a standard pressure setting of 29.92 inches of mercury (Hg) or 1013.25 hPa. This standardization is essential for ensuring that aircraft flying at high altitudes can maintain safe vertical separation.

In the United States, the transition altitude is typically set at 18,000 feet (FL180), which is why this answer is correct. Above this altitude, all aircraft must use the standard setting regardless of the local pressure. This practice maintains uniformity across different aircraft and helps air traffic controllers manage traffic effectively in the increasingly crowded airspace above 18,000 feet.

Other altitudes mentioned—FL120, FL240, and FL300—are not designated as the transition altitude and vary by region and airspace control practices. Therefore, they do not provide the standardization needed for transition to high altitude operations across a wide range of flight environments.