Which unit is used on the barometric scale to express standard atmospheric pressure?

• A. Inches of mercury
• B. PSI
• C. Pascal
• D. Newton per square meter

Answer: (A)

The barometric scale on an aircraft altimeter is calibrated in inches of mercury because it’s based on the traditional mercury barometer used to measure atmospheric pressure. Standard sea-level pressure is defined as 29.92 inches of mercury, so the instrument reads pressure as the height of a mercury column. This unit provides a direct, intuitive link between pressure and altitude readings that pilots rely on.

In contrast, PSI is a unit tied to force per area used in other contexts (like tire pressure), and Pascal (or Newton per square meter) are SI units that aren’t the conventional readout for the altimeter’s barometric scale, though they describe the same physical pressure. Converting 29.92 inHg gives about 1013 hPa or 101325 Pa, but the cockpit altimeter’s barometric scale remains inches of mercury.