Calculate voltage, current, resistance or power using Ohm's law
An Ohm's law calculator works out voltage, current, resistance or power in an electrical circuit when you know two of the values. Ohm's law is the foundational relationship in electronics: voltage equals current times resistance (V = I × R). With that single equation, and the power formulas built around it, knowing any two quantities lets you solve for the others — which is what you do constantly when designing a circuit, choosing a resistor or working out why a component is overheating.
The reason a calculator helps is that the four quantities — voltage (volts), current (amps), resistance (ohms) and power (watts) — interlock through several rearrangements of the same formulas, and it is easy to transpose them wrongly under pressure. Enter the two values you have and the tool returns the rest, correctly rearranged. A common practical use is sizing a resistor or checking that a part stays within its power rating, since power is what turns into heat. This tool computes everything in your browser.
Ohm's law states that voltage equals current times resistance: V = I × R. Rearranged, current is voltage divided by resistance, and resistance is voltage divided by current. These three forms of one equation let you find any of the three values from the other two.
Power (in watts) is voltage times current: P = V × I. Combined with Ohm's law, power can also be written as I² × R or V² ÷ R, so you can calculate it from whichever two of voltage, current and resistance you know. Power is what determines heat, so it matters for component ratings.
Voltage is measured in volts (V), current in amperes or amps (A), resistance in ohms (Ω) and power in watts (W). Keeping the units consistent — for example using amps rather than milliamps — is essential, since mixing them is a common source of wrong answers.
Power is the energy a component turns into heat. If a resistor or part dissipates more power than it is rated for, it overheats and can fail or burn out. Calculating the power lets you choose a component with an adequate rating before building the circuit.
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