When a liquid is contained in a vessel, it exerts force at all points on the sides and bottom of the vessel. The force per unit area is called intensity of pressure. Mathematically, intensity of pressure,
p = P/A
where
P = Force acting on the liquid, and
A = Area on which the force acts.

The intensity of pressure may be expressed either in N/m², N/mm² or in metres of liquid or mm of liquid. Intensity of pressure is also expressed in pascal (briefly written as Pa), such that
1 Pa= 1 N/m²; 1 kPa= 1 kN/m² and 1 MPa = 1 MN/m²= 1 N/mm²