Inches of mercury (inHg) and millimeters of mercury (mmHg) are non-SI units of pressure, based on the height of a column of mercury in a barometer. One inch of mercury is equivalent to the pressure exerted at the base of a column of mercury exactly one inch high at 0°C, whereas one millimeter of mercury corresponds to a mercury column exactly one millimeter high at the same temperature. One inch of mercury equals 25.4 millimeters of mercury, or approximately 3,386.39 Pascals. These units originated from the use of mercury barometers, invented by Italian scientist Evangelista Torricelli in the 17th century, to measure atmospheric pressure.