is a force per unit area, measured in kilopascals (kPa) or pounds per square inch (psi). Standard atmospheric pressure at sea level is 101.3 kPa (14.7 psi), or one atmosphere. Gauge pressure, as indicated by a conventional pressure gauge, is the pressure in excess of atmospheric pressure. Absolute pressure is the sum of atmospheric and gauge pressures. Pressure is also expressed in terms of the height of a column of mercury that would exert the same pressure. One atmosphere is equal to 760 mm (29.9 in) of mercury.