What should the altimeter be set to in order to determine pressure altitude before takeoff?

To determine pressure altitude before takeoff, the altimeter should be set to 29.92 inches of mercury (Hg). This setting is used because pressure altitude is defined as the height above the standard datum plane, which is determined at 29.92 inches of mercury.

When pilots set their altimeters to 29.92"Hg, they are essentially referencing a standard atmospheric pressure which allows them to obtain an accurate measure of altitude relative to a standardized environment. This standardization is crucial for ensuring that all aircraft are operating under the same altitude references, especially when flying in controlled airspace.

Other altimeter settings, such as 30.00"Hg, 28.50"Hg, or 30.25"Hg, deviate from this standard and would not provide a true representation of pressure altitude relative to sea level under standard atmospheric conditions. Each of those values would offer a different perspective of altitude, which could lead to discrepancies in navigation and safety during flight operations.