Electromotive force, or EMF, is calculated using the formula e = E / Q, where e is the EMF, E is the energy in Joules and Q is the charge. The EMF measures the potential difference between the terminals of a cell when the circuit is switched off.

Electromotive force can also be calculated using the formula, e = I(R + r), where I is the current flowing in the circuit, R is the load resistance across the circuit and r is the internal resistance of the cell. When drawing the graph of terminal potential difference against current, the y-intercept is the same as the EMF.