Voltage pushes, current flows, resistance holds back and power is the rate at which energy is spent. Ohm's law ties the first three together as V = I × R, and the power relationship P = V × I joins on the fourth. Knowing any two therefore pins down the whole circuit, which is why this page takes any pair rather than a fixed one.

Using it in three steps

1. 1 Type the two values you know. It does not matter which two.
2. 2 The remaining two appear immediately, each labelled as calculated so you can tell them from your own figures.
3. 3 To try a different pair, press Clear all - editing over a calculated box on its own can leave you solving the wrong circuit.

Six pairs, one circuit

There are six ways to pick two values out of four, and all six lead to the same answer for a given circuit. Twelve volts with two amps, or six ohms with twenty-four watts, or two amps with six ohms - each pair describes the same lamp. That consistency is a useful check on a measurement: if two different pairs disagree, one of your readings is wrong.

Where the law stops applying

Ohm's law assumes resistance stays constant, and plenty of real components refuse to. A filament lamp gets hotter and more resistant as current rises; a diode barely conducts until it suddenly does. These calculations still describe a working point, but you cannot take one measurement and scale it. For alternating current, resistance also becomes impedance, and the phase between voltage and current starts to matter.