Ohm's Law Calculator
Calculate voltage, current, resistance and power from any two known values.
For resistive DC circuits (and AC circuits with purely resistive loads). Mains electricity is dangerous — leave wiring work to a qualified electrician.
About this tool
Choose which two values you know — voltage, current, resistance or power — and enter them. The calculator applies Ohm's law (V = I × R) and the power formula (P = V × I) to find the other two, for resistive DC circuits.
Frequently asked questions
Ohm's law says voltage equals current times resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. A 12 V supply across a 6 Ω resistor drives 12 ÷ 6 = 2 A.
Power is voltage times current: P = V × I. Substituting Ohm's law gives P = I² × R and P = V² ÷ R. At 12 V and 2 A the power is 24 W.
I = P ÷ V = 60 ÷ 230 ≈ 0.26 A. On a 120 V supply the same 60 W bulb draws 60 ÷ 120 = 0.5 A.
It applies directly to resistive loads such as heaters and incandescent bulbs. Motors, transformers and electronics add reactance, so you need impedance and power factor instead of plain resistance.