It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity.

In the United States, Canada[2] and Japan, these altimeter readings are provided in inches of mercury, but most other nations use hectopascals.

Aircraft operating at higher altitudes (at or above what is called the transition altitude, which varies by country) set their barometric altimeters to a standard pressure of 29.92 inHg (1 atm = 29.92 inHg) or 1013.25 hPa (1 hPa = 1 mbar) regardless of the actual sea level pressure.

However, the inch of mercury is still used today in car performance modification to measure the amount of vacuum or pressure within the engine's intake manifold.

Inches of mercury was the usual unit of pressure measurement in railway vacuum brakes.