In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced (the volume of the immersed object will be equal to the volume of the displaced fluid).

An object that sinks displaces an amount of fluid equal to the object's volume. Thus buoyancy is expressed by Archimedes' Principle which states that when this happens, the weight of the object is reduced by its volume times the density of the fluid. If the weight of the object is less than this quantity, it will float, if more it will sink. The amount of fluid displaced is directly related (via Archimedes' Principle) to its weight.

In the case of an object which sinks the amount of fluid displaced will be equal in volume to the immersed object, whereas in the case of an object which floats the amount of fluid displaced will be equal in weight to the displacing object.