In fluid mechanics, hydrostatic equilibrium or hydrostatic balance (also known as hydrostasy) is the condition of a fluid or plastic solid at rest. This occurs when external forces such as gravity are balanced by a pressure-gradient force.

Hydrostatic equilibrium happens when the pull of gravity is balanced by a pressure gradient which creates a pressure-gradient force in the other direction. The balance of these two forces is known as the hydrostatic balance. It flattens the surface of undisturbed liquids.