When a force acts on a surface, its effect depends on how that force is distributed. Pressure measures the force applied perpendicular to a surface divided by the area over which it acts.

Formally, pressure is defined as

where $F_\perp$ is the force perpendicular to the surface and $A$ is the area. If a force acts at an angle, only the normal component $F_{\perp}$ enters the calculation.

The SI unit is the pascal (Pa), with $1 \text{ Pa} = 1 \text{ N/m}^2$. Since a pascal is small, pressures are often given in kilopascals (kPa) or atmospheres (atm).