The Gran plot is based on the Nernst equation which can be written as
where E is a measured electrode potential, E is a standard electrode potential, s is the slope, ideally equal to RT/nF, and {H} is the activity of the hydrogen ion. The expression rearranges to
depending on whether the electrode is calibrated in millivolts or pH. For convenience the concentration, [H], is used in place of activity. In a titration of strong acid with strong alkali, the analytical concentration of the hydrogen ion is obtained from the initial concentration of acid, C and the amount of alkali added during titration.
where v is the initial volume of solution, c is the concentration of alkali in the burette and v is the titre volume. Equating the two expressions for [H] and simplifying, the following expression is obtained
A plot of  against v will be a straight line. If E and s are known from electrode calibration, where the line crosses the x-axis indicates the volume at the equivalence point, . Alternatively, this plot can be used for electrode calibration by finding the values of E and s that give the best straight line.