Millimeter of Mercury (mmHg) and Inch of Mercury (inHg) are two units of measurement used to express pressure, particularly in the context of atmospheric pressure and blood pressure. The millimeter of mercury is a metric unit, where 1 mmHg is equivalent to the pressure exerted by a column of mercury 1 millimeter high. On the other hand, the inch of mercury is a US customary unit, where 1 inHg is equivalent to the pressure exerted by a column of mercury 1 inch high. Both units are widely used in scientific and medical applications, such as measuring blood pressure, atmospheric pressure, and pressure in industrial processes.