Cell EMF Calculator

01 Set up your calculation

1. Enter the reduction potential at the selected cathode.
2. Enter the reduction potential at the selected anode without reversing its tabulated sign.
3. Read the difference and check that both potentials refer to the same conditions and reference electrode.

02 The relationship

For the assumptions behind these results, read the explanation below. You can also explore Electrolysis and Gibbs Free Energy.

03 Use one sign convention throughout

Electrode tables commonly report reduction potentials. Although oxidation occurs at the anode, enter its listed reduction potential here; the subtraction supplies the required sign reversal. Flipping its sign first and then subtracting would reverse it twice.

Both values must share the same reference electrode. Potentials reported relative to different reference electrodes require an appropriate conversion before comparison.

04 Voltage is not total energy or operating output

Cell potential measures energy per charge, so multiplying a half-reaction to balance electrons does not multiply its potential. Total reaction free energy also depends on how many moles of electrons are transferred.

Standard potentials describe standard conditions. Real cell voltage can change with composition, temperature, internal resistance, polarization, and current draw. This difference calculation does not model loaded battery performance. A measured terminal voltage should be compared with a thermodynamic estimate only after accounting for the actual operating conditions.