Ohm’s law ties together the four quantities that describe a simple circuit — voltage, current, resistance and power. Know any two and you can find the other two. That is the whole trick, and it is the foundation of nearly every electrical calculation you will ever do.
The law itself
Voltage equals current times resistance: V = I × R. Rearranged, current is I = V ÷ R and resistance is R = V ÷ I. In plain terms: push (voltage) drives flow (current) against opposition (resistance). More push or less opposition means more current.
Adding power
Power is voltage times current: P = V × I. Fold in Ohm’s law and you also get P = I² × R and P = V² ÷ R — so whichever pair of values you know, there is a formula that reaches power. For example, 120 V at 2 A is 240 W (230 V at 2 A is 460 W).
| You know | Current I | Power P |
|---|---|---|
| V and R | V ÷ R | V² ÷ R |
| V and I | — | V × I |
| I and R | — | I² × R |
Ohm's Law Calculator
Enter any two of volts, amps, ohms or watts and get the other two instantly.
The Ohm’s law triangle (and wheel)
The classic memory aid: put V on top with I and R below it. Cover the value you want and the layout tells you the sum — cover V and you get I × R, cover I and you get V ÷ R. A second triangle with P over V and I does the same for power. The fuller Ohm’s law wheel folds all eight formulas into one diagram.
Does it work for AC?
Directly for DC and for purely resistive AC loads such as heaters and incandescent lamps. On AC circuits with motors, transformers or electronics, current and voltage fall out of step, so real power is V × I × power factor — the V × I figure is then the apparent power in VA, and you apply the power factor separately. For that step, reach for the kVA or kW-to-amps calculators.