Ohm's Law Calculator
Enter any two of voltage, current, resistance and power. Leave the others blank and the calculator fills them in, telling you which pair it solved from.
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The formula
V = I × R · P = V × I = I²R = V²/R
Two equations relate all four quantities, so any two known values determine the other two. The calculator picks the pair you filled in and works outwards.
What to watch out for
Ohm's law is two equations doing four jobs:
- V = I × R — voltage is current times resistance.
- P = V × I — power is voltage times current.
Substituting one into the other gives P = I²R and P = V²/R, which is why any two of the four values are enough. The I²R form is the one that matters in practice: doubling the current quadruples the heat, which is why undersized conductors fail thermally rather than electrically.
This applies to DC and to purely resistive AC loads. For anything reactive — motors, transformers, switched-mode supplies — power is V × I × power factor, and the power factor calculator covers that.
Frequently Asked Questions
What is Ohm's law?
Voltage equals current times resistance: V = I × R. It means that for a fixed resistance, current rises in proportion to voltage.
How do I find current from watts and volts?
Divide: I = P ÷ V. A 60 W bulb on 240 V draws 0.25 A. Enter the power and voltage above and the current appears.
Why does power go up with the square of current?
Because P = I²R. The voltage across a resistance is itself proportional to current, so both terms rise together. Doubling current through the same resistor produces four times the heat.
Does Ohm's law work for AC?
For resistive loads, yes. For reactive loads you need impedance rather than resistance, and real power needs the power factor as well.