The Hg/HgO electrode is usually utilized as a reference electrode in alkaline solution such as for development of an alkaline hydrogen electrode. The reference electrode provides a suitable reference point but is available from few commercial vendors and suffers from inadequate documentation on potential in varying electrolytes. A new numerical method uses activity, activity coefficients, and a few correlated empirical equations to determine the potential values in both dilute and concentrated sodium hydroxide solutions at temperatures of 0–90°C and at concentrations of 0.100–12.8 mol k g H 2 O − 1 . The computed potentials of the Hg/HgO electrodes versus a normal hydrogen electrode (NHE) at 25°C and 1atm are 0.1634V for 0.100m, 0.1077V for 1.00m, and 0.0976V for 1.45m NaOH solutions. The Hg/HgO reduction potential further changes to −0.0751V versus NHE and hydrogen evolution potential changes to −0.9916V versus NHE in a solution of 30.0wt.% NaOH at 80°C. The calculated values are compared with the measured data at 25 and 75°C. The experimental data agree well with the numerical values computed from the theoretical and empirical equations.