Kirchhoff's loop rule states that the sum of the potential drops around a closed loop in a circuit is equal to the voltage supplied to the loop. This rule arises from conservation of energy, which states that energy cannot be created nor destroyed but is instead transferred. Voltage sources, such as a battery or a capacitor, supply the energy, and components such as resistors create the potential drops.

Mathematically, Kirchhoff's rule is represented as stating that the sum of all voltages in the loop is equal to zero. Because his rule is a simplification of Faraday's law of induction, the loop rule only holds under the assumption that there is no fluctuating magnetic field interacting with the circuit, when the circuit is under DC conditions, and with AC circuits at lower frequencies where the wavelengths are very high.

If magnetic fields are present, voltages will be induced to the circuit that are not from a battery or capacitor and the rule will not hold. In high-frequency AC circuits, the current through a resistor may not immediately flow out the other end and due to the fluctuations in the current a voltage will be induced in the circuit due to the changing magnetic fields. Kirchhoff's rule is useful in electrical engineering in order to easily solve multi-loop circuits.