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Ohm's Law Calculator

Solve for volts, amps, ohms or watts from any two known values.

Waiting for inputs

Enter any two values above and the remaining two are solved instantly.

What this assumes

Direct current, or alternating current at unity power factor.
Resistance is treated as constant and purely resistive.
For AC circuits with motors or electronics, apply the power factor separately.

Important

Circuit calculations do not replace a design review. Installed work must comply with the National Electrical Code as adopted and amended by your jurisdiction.

Understanding the result

What Ohm's law tells you

Ohm's law fixes the relationship between voltage, current and resistance: V = I × R. Combined with the power formula P = V × I, knowing any two of the four values determines the other two. It is the foundation of every load, conductor and protection calculation that follows.

Where it stops being exact

The law describes purely resistive DC behavior. On AC circuits with motors, transformers or switching power supplies, current and voltage fall out of phase and real power is less than volts times amps. In those cases you need power factor, and apparent power in VA rather than watts.

Common questions

Can I use Ohm's law on a 240V AC circuit?

Yes, for resistive loads such as water heater elements and baseboard heat. For motors, compressors and electronics, the reactive component means you also need the power factor to get real power.

Why does my measured current differ from the calculation?

Real conductors have resistance, connections add resistance, and supply voltage sags under load. A measurement within a few percent of the calculation is normal; a large gap usually indicates a loose connection or an undersized conductor.

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