In a static (at rest) fluid, pressure increases with depth. Deeper layers support the weight of all fluid above them.

Consider a horizontal surface at depth $h$ below the fluid surface. The fluid column above has height $h$ and cross-sectional area $A$.

The volume of the fluid column is

Since density is $\rho = m/V$, the mass of this column is

The weight of the column acts downward on the surface:

Pressure is force per unit area. Dividing by $A$:

This is the hydrostatic pressure formula:

Here $\rho$ is fluid density, $g$ is gravitational acceleration, and $h$ is depth. This assumes an incompressible, static fluid.

A key insight: pressure depends only on depth. The container shape and total fluid volume do not matter. At depth $h$, the pressure $P = \rho g h$ acts equally in all directions—on floors, walls, and any submerged object.