Last updated October 10, 2026 · Report an error
Ohm's Law is the fundamental relationship between voltage, current, and resistance in an electrical circuit.
This calculator finds whichever value you leave blank — voltage, current, or resistance — from the other two, and also reports power.
How it works
Enter any two of voltage, current, and resistance, leaving the third blank. The calculator applies Ohm's Law \(V = I \times R\) to solve for the missing value, and always reports power as \(P = V \times I\).
- Enter voltage (V) — leave blank to solve for this.
- Enter current (A) — leave blank to solve for this.
- Enter resistance (Ω) — leave blank to solve for this.
- Click Calculate to see your results.
Examples
12V, 4Ω resistor
A 12-volt source across a 4-ohm resistor draws 3 amps of current, consuming 36 watts of power.
Who should use it
- Checking circuit calculations for electronics coursework or hobby projects.
- Sizing a resistor or estimating current draw for a simple circuit.
Industry applications
- Electrical engineering education
- Electronics hobbyist and DIY circuit design
Advantages
- Solves for any of the three Ohm's Law quantities from the other two.
- Automatically reports power alongside the calculated value.
Limitations
- Assumes a simple resistive DC circuit — doesn't account for reactance in AC circuits.
Common mistakes to avoid
- Leaving more than one field blank — the calculator needs exactly two known values to solve for the third.
- Mixing up units — this calculator expects volts, amps, and ohms directly, not millivolts or kilo-ohms.
Best practices
- Double-check which value you actually need before reading the result — the calculated value updates based on which field was left blank.
Tips
- If you know the wattage of a device and its operating voltage, you can find its current draw first (I = P/V), then use that here to find resistance.
Frequently asked questions
Yes, with no signup and no limit on how many calculations you run.
The calculator uses the values as given and reports the power — it doesn't need to solve for anything since all three are already known.
It states that current through a conductor is directly proportional to voltage and inversely proportional to resistance: V = IR.
Power (in watts) is calculated as voltage times current (P = VI), and is useful for sizing components and estimating energy use.