 In equilibrium, the forward binding transition R + L → RL should be balanced by the backward unbinding transition RL → R + L. That is, 

where [R], [L] and [RL] represent the concentration of unbound free receptors, the concentration of unbound free ligand and the concentration of receptor-ligand complexes.The binding constant "K"a is defined by

An often considered quantity is the dissociation constant "K"d ≡ , which has the unit of concentration, despite the fact that strictly speaking, all association constants are unitless values.The inclusion of units arises from the simplification that such constants are calculated solely from concentrations, which is not the case.