Millimeters of mercury (mm Hg) is the unit of barometric pressure in the metric system. Barometric pressure, also called air pressure or atmospheric pressure, is a measure of the weight of the atmosphere above the surface of the Earth at a given point.
The unit of barometric pressure derives from measuring the height of a column of mercury in a glass tube. This type of barometer, invented by Evangelista Torricelli in 1643, remains the most common instrument for measuring atmospheric pressure. Millimeters of mercury can be converted to the British and American unit of barometric pressure, inches of mercury, by multiplying the figure by 25.4.