Inch of mercury

Inches of mercury, inHg or "Hg is a non-SI unit for pressure. It is still widely used for barometric pressure in weather reports and aviation in the United States, but is considered somewhat outdated elsewhere.

It is defined as the pressure exerted by a column of mercury of 1 inch in height at 32 °F (0 °C) at the standard acceleration of gravity.


 * 1 inHg = 3,386.389 pascals at 0 °C.

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 or 1,013.2 hPa (1 hPa = 1 mbar) regardless of the actual sea level pressure, with inches of mercury used in the U.S. and Canada. The resulting altimeter readings are known as flight levels.

Piston engine aircraft with constant-speed propellers also use inches of mercury to measure manifold pressure, which is indicative of engine power produced.

In older literature, an inch of mercury based on the height of a column at 60 °F (15.6 °C) was common.
 * 1 inHg60 °F = 3,376.85 Pa