Ohm's Law Calculator: Voltage, Current, Resistance & Power
Enter any two of the four values and the other two are calculated instantly. Leave the two you don't know blank. Everything runs in your browser — nothing is uploaded anywhere.
Formulas: V = I × R · I = V ÷ R · R = V ÷ I · P = V × I = I² × R = V² ÷ R
Worked example: a 12 V supply across a 6 Ω resistor gives I = 12 ÷ 6 = 2 A, and P = 12 × 2 = 24 W. The resistor must be rated above 24 W or it will overheat — in practice, choose a rating with comfortable margin.
Ohm's law only holds for resistive, linear loads at a steady temperature. For AC circuits with motors, transformers or capacitors, use the three-phase power calculator instead, because reactance and power factor change the picture.
Frequently asked questions
What is Ohm's law in simple terms?
Voltage equals current times resistance (V = I × R). Raise the voltage or lower the resistance and more current flows; the power dissipated as heat is V × I.
Why does the calculator ask for exactly two values?
Any two of voltage, current, resistance and power determine the other two through Ohm's law and the power formulas. With one value the answer is under-determined; with three, the third may contradict the other two.
Does Ohm's law work for AC circuits?
Only for purely resistive AC loads. With inductance or capacitance, impedance replaces resistance and power factor must be included — see the three-phase power calculator for AC power.