压强是分布在特定作用面上之力量与该面积的比值。换句话说，是作用在与物体表面垂直方向上的每单位面积的力量大小。计式压强是相较于该地之大气压的压强。虽然压强可用任意之力量单位与面积单位进行测量，但是压强的国际标准单位（每单位平方公尺的牛顿）也被称作帕斯卡。

一般以英文本母「p」表示。压强与力和面积的关系如下：

根据上述公式，可以推导出计算液体压强的公式：

即：

其中：

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure.

Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre; similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and US customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure and the torr is defined as ⁄₇₆₀ of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.