Ohm's Law Calculator
Fill in any two of voltage, current, resistance and power, leave the others blank, and the calculator solves for the rest using Ohm's law (V = I × R) and the power formula (P = V × I).
Your numbers
Result
Enter your numbers to see the result.
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The formulas
- V = I × R
- P = V × I = I² × R = V² ÷ R
With any two values known, the other two follow. For example, V and R give I = V ÷ R and P = V² ÷ R. These formulas are exact for DC and for resistive AC loads (heaters, incandescent lamps). Motors and other reactive AC loads also need power factor.
Worked example
A 120 V circuit through a 24 Ω heating element draws 120 ÷ 24 = 5 A and uses 120 × 5 = 600 W.
Frequently asked questions
What if I enter three values?
Leave exactly two filled in. If more are given, the first two in the order voltage, current, resistance, power are used.
Does it work for AC?
For resistive loads, yes. For motors or transformers use real power with power factor on the three-phase power calculator.
Related calculators
This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.