In a fluid at rest, the pressure at any point beneath the fluid surface depends solely on the depth, not on the container's shape or size. This principle, known as hydrostatic pressure, arises because, in stationary fluids, there is no acceleration, meaning the forces within the fluid balance out. Only vertical forces, caused by the weight of the fluid above, contribute to pressure changes with depth.

When measuring pressure at two different levels within the fluid, the difference in pressure is due to the weight of the fluid between these points. This difference can be expressed in terms of pressure head, a concept that links the depth of the fluid to the pressure it exerts. The pressure head is directly proportional to the vertical distance or depth between two points. The deeper the point within the fluid, the greater the pressure due to the weight of the fluid above it.

If the fluid's surface is the reference point, where pressure is often atmospheric, then the pressure at any depth below the surface equals the atmospheric pressure plus the additional pressure exerted by the fluid above that point. This relationship underlies many practical applications, such as understanding underwater pressures, determining water pressure in tanks, and designing structures to withstand fluid forces.