Ohm's Law Calculator

Solve for voltage, current, resistance, or power — enter any two to find the other two.

Enter any two values — the other two are calculated automatically.

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How it works

Enter any two of the four electrical quantities — voltage (V), current (I), resistance (R), or power (P). Ohm's Law (V = I × R) and the power equation (P = V × I) together allow solving for all four from any two. The tool picks the appropriate formula pair, solves, and displays all four values with their units. All calculation runs locally in your browser.


              V = IR   P = VI   P = I²R   P = V²/R

Common use cases

  • Finding the correct resistor value for an LED circuit at a given voltage and current.
  • Calculating the current draw of an appliance from its wattage and supply voltage.
  • Checking whether a wire or component can handle the power dissipation in a circuit.

Frequently asked questions

Does this work for AC circuits?

Ohm's Law applies to DC and to resistive AC loads. For reactive AC circuits with capacitors or inductors, you need impedance (Z) instead of pure resistance (R). This tool covers the resistive case only.

What units does this use?

Standard SI units: volts (V), amperes (A), ohms (Ω), and watts (W). Enter values in these base units; use decimal notation for milliamps (e.g. 0.02 A for 20 mA).