When is indicated altitude the same as true altitude?

Indicated altitude is the altitude shown on the altimeter when it is set to the current pressure at sea level, and it aligns with true altitude under specific conditions. The correct context is that indicated altitude equals true altitude specifically at sea level when standard atmospheric pressure is in effect, which is defined as 29.92 inches of mercury (Hg) at a temperature of 15 degrees Celsius.

At sea level under these standard conditions, the pressure is equal to the expected sea-level pressure, allowing the altimeter to accurately reflect actual elevation above sea level. This means that when a pilot is operating an aircraft at sea level and the altimeter is correctly calibrated to standard pressure, indicated altitude will match true altitude.

In contrast, indicated altitude at high elevations or in varying atmospheric conditions—such as non-standard pressure or temperature—will result in discrepancies between indicated and true altitude. Similarly, a miscalibrated altimeter or colder-than-standard air creates scenarios where indicated altitude does not equal true altitude.