The force of pressure on the bottom of the object is greater than that on the top. The top and bottom pressures both increase with the depth in the fluid; their difference is the buoyancy.

Formally, the buoyant force can be defined as the product of the weight of the fluid displaced by the object , the acceleration due to gravity , and the object's volume . In symbols, .

If the buoyancy is less than the weight of the object, the object will sink; otherwise it will rise to the top of the fluid and float. In symbols, the object sinks if , where is the weight of the object, and rises if .

Notice that the object's volume in this example is constant (and therefore so is the volume of the displaced fluid), but you can vary the weight.