If carbon dioxide, carbonic acid, hydrogen ions, bicarbonate and carbonate are all dissolved in water, and at chemical equilibrium, their equilibrium concentrations are often assumed to be given by:
where the subscript eq denotes that these are equilibrium concentrations, K is the equilibrium constant for the reaction  +   H +  (i.e. the first acid dissociation constant for carbonic acid), K is the equilibrium constant for the reaction   H +  (i.e. the second acid dissociation constant for carbonic acid), and DIC is the (unchanging) total concentration of dissolved inorganic carbon in the system, i.e. [] + [] + []. K, K and DIC each have units of a concentration, e.g. mol/L.
A Bjerrum plot is obtained by using these three equations to plot these three species against , for given K, K and DIC. The fractions in these equations give the three species' relative proportions, and so if DIC is unknown, or the actual concentrations are unimportant, these proportions may be plotted instead.
These three equations show that the curves for  and  intersect at , and the curves for  and  intersect at . Therefore, the values of K and K that were used to create a given Bjerrum plot can easily be found from that plot, by reading off the concentrations at these points of intersection. An example with linear Y axis is shown in the accompanying graph. The values of K and K, and therefore the curves in the Bjerrum plot, vary substantially with temperature and salinity.