The relation between the equilibrium constant of a given chemical reaction and the associated free energy is an issue well studied in chemistry books, but when the reaction involves changes in the number of components in the system, as is the case in binding, things become a little more obscure since one needs to define the so-called standard state. This is reflected in the literature, especially in computational studies of binding, where contradicting approaches are followed when treating this problem. In this work, we present a detailed and unifying explanation of the concepts involved and derive the necessary relations to convert a binding free energy from an arbitrary state to some given standard state. This is done in three independent ways, from the point of view of (1) the dimensions of the quantities involved, (2) the energy and entropy of the molecules, and (3) their chemical potentials.