Pressure is defined as the force per unit area. It can be applied either to a solid or a liquid interface at a point perpendicular to the surface and is represented by the height of the column that exerts a pressure at its base. Depending on the fluid used, the height of a column of fluid will differ because its density and therefore the pressure it exerts will differ.

Mercury (Hg) is a very dense fluid (13.5951 g/cm³) and is often used to calibrate pressure gradients and differentials. Normal atmospheric pressure can support 760mmHg. One millimeter of Hg therefore represents approximately 1/760 of an atmosphere and is a useful measure for comparing pressures. This measure is often referred to as a Torr, after the Italian physicist Torricelli. The unit mmHg is very useful in measurements for biologic systems. It is the conventional unit used to measure blood pressure.