Inch of mercury
It is the pressure exerted by a column of mercury of 1 inch in height at the standard acceleration of gravity. Conversion to metric units depends on the temperature of Mercury, and hence its density; typical conversion factors are:
Aircraft altimeters measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground. Within the U.S. and Canada, these readings are provided in inches of mercury. Ground readings vary with weather and along the route of the aircraft as it travels, so current readings are relayed periodically by air traffic control. 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. 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 automobile racing, particularly USAC and CART Indy car racing, inches of mercury was the unit used to measure turbocharger inlet pressure. However, the inch of mercury is still used today in car performance modification to measure the amount of vacuum within the engine's intake manifold. This can be seen on boost/vacuum gauges.
In air conditioning and refrigeration, inHg is often used to describe "inches of mercury vacuum", or pressures below 0 psig, for recovery of refrigerants from air conditioning and refrigeration systems, as well as for leak testing of systems while under a vacuum, and for dehydration of refrigeration systems. The low side gauge in a refrigeration gauge manifold indicates pressures below 0 psig in "inches of mercury vacuum" (inHg), down to a 30 inHg vacuum.
In older literature, an "inch of mercury" is based on the height of a column of mercury at 60 °F (15.6 °C) 
- 1 inHg60 °F = 3376.85 Pa
In English units: 1 inHg = 0.491 098 psi, or 2.036 254 inHg = 1 psi.