Archimede's principle

Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

Source:Wikipedia en

Formula

Buoyancy : {\vec {P}}_{{{\rm {A\,}}}}=-\,M_{{{\rm {f\,}}}}{\vec {g}} where Mf is the mass of the fluid displaced in kg and g the intensity of the gravity field N.kg-1. F is expressed in newton (N).