What is the difference between altitude and flight level?

What is the difference between altitude and flight level?

• A. Flight level is the actual altitude; altitude is standard pressure.
• B. Altitude is height above mean sea level; flight level is pressure-based altitude (QNH vs standard pressure 29.92).
• C. Altitude is the same as flight level.
• D. Altitude is height above ground; flight level is height above sea level.

Answer: (B)

The difference comes from how the altimeter is set. Altitude is the height above mean sea level and is read with the altimeter set to the local pressure (QNH). Flight level is pressure altitude, read with a standard pressure setting (29.92 inHg / 1013 hPa). Because pressure changes with weather and location, using the local QNH ties the reading to height above sea level, while using the standard pressure provides a fixed reference plane for consistent separation in higher airspace. That’s why flight levels are used above certain altitudes, giving uniform vertical spacing, whereas altitude above MSL is used lower down with the local pressure setting.

The difference comes from how the altimeter is set. Altitude is the height above mean sea level and is read with the altimeter set to the local pressure (QNH). Flight level is pressure altitude, read with a standard pressure setting (29.92 inHg / 1013 hPa). Because pressure changes with weather and location, using the local QNH ties the reading to height above sea level, while using the standard pressure provides a fixed reference plane for consistent separation in higher airspace. That’s why flight levels are used above certain altitudes, giving uniform vertical spacing, whereas altitude above MSL is used lower down with the local pressure setting.