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Ohm's Law & Power Calculator

Enter any two of voltage, current, resistance and power — the other two are calculated instantly.

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Fill in any two of the four fields; the other two are calculated.

Two In, Two Out

Ohm's law (V = I·R) and the power law (P = V·I) tie the four electrical quantities together, so any two determine the other two. All six combinations are covered: from V and I comes R = V/I and P = V·I; from P and R comes V = √(P·R) and I = √(P/R), and so on — the classic Ohm's law wheel, without having to remember which slice to read.

Values assume DC or resistive AC loads. For reactive AC loads (motors, ballasts), apparent power and power factor enter the picture — see our motor current calculator.

Frequently Asked Questions

Which two values should I enter?

Any two you actually know. Enter exactly two of the four fields and the tool computes the rest; change a value and it recomputes. Clear a field to swap which pair drives the math.

Does this work for AC?

For purely resistive AC loads (heaters, incandescent lamps), yes — use RMS values. For motors and electronics, power factor makes P < V·I; use the motor current calculator instead.

What units are used?

Volts, amps, ohms and watts. For milliamps or kilohms, convert first (1 mA = 0.001 A, 1 kΩ = 1000 Ω).

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