气压的国际单位制是帕斯卡（或简称帕，符号是Pa），泛指是气体对某一点施加的流体静力压强，来源是大气层中空气的重力，即为单位面积上的大气压力。在一般气象学中人们用千帕斯卡（KPa）、或使用百帕（hPa）作为单位。测量气压的仪器叫气压表。其它的常用单位分别是：巴（bar，1bar=100,000帕）和公分水银柱（或称公分汞柱）。在海平面的平均气压约为101.325千帕斯卡（76公分水银柱），这个值也被称为标准大气压。另外，在化学计算中，气压的国际单位是「atm」。一个标准大气压即是1atm。1个标准大气压等于101325帕，1.01325巴，或者76公分水银柱。

Atmospheric pressure, sometimes also called barometric pressure, is the pressure exerted by the weight of air in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air one square centimeter in cross-section, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kg and weight of about 10.1 N (2.27 lb_f) That force over one square centimeter is a pressure of 10.1 N/cm or 101000 N/m. (A column one square inch in cross-section would have a weight of about 14.7 lb, or about 65.4 N.)