Ohm's Law Calculator
Solve any DC circuit from two known values.
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Enter two of voltage, current, resistance or power and get the other two instantly, with the formulas used.
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How to Use
- 1 Enter two known values — Fill in exactly two of the four fields: voltage in volts (V), current in amperes (A), resistance in ohms (Ω) or power in watts (W).
- 2 Read the derived values — The two quantities you did not enter are calculated instantly and highlighted in the results.
- 3 Check the formulas used — The calculator shows exactly which equations it applied, so you can follow or check the working.
- 4 Copy or share your results — Copy any single value, use "Copy All" for the full set, or share the result with a link.
Frequently Asked Questions
What is Ohm's law?
Ohm's law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I.
How do I calculate electrical power?
Power is voltage times current: P = V × I. Combined with Ohm's law this also gives P = I² × R and P = V² ÷ R, so power can be found from any two of voltage, current and resistance.
Which two values do I need to enter?
Any two of the four. Voltage + current, voltage + resistance, voltage + power, current + resistance, current + power, or resistance + power all work — the calculator picks the right formulas for the pair you give it.
Why do I get an error when I enter zero?
Some combinations divide by the value you entered. For example, resistance from voltage and current is R = V ÷ I, which is undefined when the current is zero. Enter a non-zero value or use a different pair.
Does Ohm's law work for AC circuits?
This calculator assumes a DC circuit with a purely resistive load. In AC circuits you must use impedance (Z) instead of resistance, and real power also depends on the power factor, so results here are only an approximation for AC.