Ohm's Law Calculator
Calculate resistance and power from voltage and current.
Try an example
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About this calculator
How This Ohm's Law Calculator Works
Enter any two of voltage, current, and resistance, and the calculator solves for the third, plus the power delivered.
The Formula
V = IR — Ohm's law relates voltage (V, in volts), current (I, in amps), and resistance (R, in ohms). Rearranged, resistance is R = V ÷ I. Power delivered is P = VI, measured in watts.
Why This Matters in Practice
Ohm's law is the single most fundamental relationship in basic electronics — it's how you size a resistor to protect an LED, figure out how much current a device will draw at a given voltage, or estimate a component's power dissipation (and whether it'll overheat) before building a circuit.
Worked example
At 12 volts and 2 amps, resistance is 12 ÷ 2 = 6 ohms, and power delivered is 12 × 2 = 24 watts.
Frequently asked questions
What is Ohm's law?
It relates voltage, current, and resistance: V = IR. Rearranged, resistance is R = V/I.
How is power calculated?
Power equals voltage times current (P = VI), measured in watts.
What if I know resistance instead of current?
Rearrange the formula: current = voltage ÷ resistance. This calculator specifically solves for resistance and power from voltage and current.
Does this work for AC circuits?
Ohm's law applies directly to resistive DC circuits; AC circuits with reactive components (capacitors, inductors) need impedance instead of plain resistance.