What is altitude measured in?

Altitude is measured in feet, relative to sea level, because this standard provides a consistent and universally understood reference for navigation and aviation safety. Measuring altitude this way allows pilots and air traffic control to ensure that aircraft maintain safe distances from terrain and each other, especially in areas with varying topography.

This standardization is crucial for maintaining regulated airspace and ensuring that all aircraft can communicate effectively, regardless of their location. While other measurements, such as meters or relative ground measurements, may be used in different contexts, the aviation industry predominantly relies on feet above sea level for operational consistency and safety. This is part of international aviation standards, allowing for a clear and reliable understanding of an aircraft's altitude.