The voltage (electromotive force E) produced by a galvanic cell can be estimated from the standard Gibbs free energy change in the electrochemical reaction according to:
where ν is the number of electrons transferred in the balanced half reactions, and F is Faradays constant. However, it can be determined more conveniently by the use of a standard potential table for the two half cells involved. The first step is to identify the two metals and their ions reacting in the cell. Then one looks up the standard electrode potential, E, in volts, for each of the two half reactions. The standard potential of the cell is equal to the more positive E value minus the more negative E' value.
For example, in the figure above the solutions are CuSO and ZnSO. Each solution has a corresponding metal strip in it, and a salt bridge or porous disk connecting the two solutions and allowing  ions to flow freely between the copper and zinc solutions. To  calculate the standard potential one looks up copper and zinc's half reactions and finds:
:Cu + 2   Cu  E = +0.34 V
:Zn + 2   Zn  E = −0.76 V
Thus the overall reaction is:
:Cu + Zn  Cu + Zn
The standard potential for the reaction is then +0.34 V − (−0.76 V) = 1.10 V. The polarity of the cell is determined as follows. Zinc metal is more strongly reducing than copper metal because the standard (reduction) potential for zinc is more negative than that of copper. Thus, zinc metal will lose electrons to copper ions and develop a positive electrical charge. The equilibrium constant, K, for the cell is given by:
where F is the Faraday constant, R is the gas constant and T is the temperature in kelvins. For the Daniell cell K is approximately equal to . Thus, at equilibrium, a few electrons are transferred, enough to cause the electrodes to be charged.
Actual half-cell potentials must be calculated by using the Nernst equation as the solutes are unlikely to be in their standard states:
where Q is the reaction quotient. When the charges of the ions in the reaction are equal, this simplifies to:
where {M} is the activity of the metal ion in solution. In practice concentration in mol/L is used in place of activity. The metal electrode is in its standard state so by definition has unit activity. The potential of the whole cell is obtained as the difference between the potentials for the two half-cells, so it depends on the concentrations of both dissolved metal ions. If the concentrations are the same, and the Nernst equation is not needed under the conditions assumed here.
The value of 2.303 is , so at 25 °C (298.15 K) the half-cell potential will change by only 0.05918 V/ν if the concentration of a metal ion is increased or decreased by a factor of 10.
These calculations are based on the assumption that all chemical reactions are in equilibrium. When a current flows in the circuit, equilibrium conditions are not achieved and the cell voltage will usually be reduced by various mechanisms, such as the development of overpotentials. Also, since chemical reactions occur when the cell is producing power, the electrolyte concentrations change and the cell voltage is reduced. A consequence of the temperature dependency of standard potentials is that the voltage produced by a galvanic cell is also temperature dependent.