Ohm's Law Calculator
Enter any two of voltage, current, resistance and power, and get the other two.
- Voltage
- Current
- Resistance
- Power
- Solved with
- Power
Saved Circuits
Items you add are saved in this browser.
| Name | Voltage | Current | Resistance | Power | Remove |
|---|
Ohm's Law and the Power Formulas
Ohm's law ties together voltage, current and resistance in a circuit: the current through a resistor equals the voltage across it divided by its resistance. Add the power formula, watts equal volts times amps, and knowing any two of the four values gives you the other two.
Worked Example
A 12 V battery drives a 6 Ω heating element. The current is 12 ÷ 6 = 2 A, and the power is 12 × 2 = 24 W. Check it another way: I² × R = 2² × 6 = 24 W.
Every Combination
| You Know | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| V and I | – | – | V ÷ I | V × I |
| V and R | – | V ÷ R | – | V² ÷ R |
| V and P | – | P ÷ V | V² ÷ P | – |
| I and R | I × R | – | – | I² × R |
| I and P | P ÷ I | – | P ÷ I² | – |
| R and P | √(P × R) | √(P ÷ R) | – | – |
Tips
Ohm's law works directly for DC circuits and for resistive AC loads like heaters and incandescent bulbs. Motors, transformers and electronics in AC circuits also have reactance, so the simple formulas give only an estimate; use the RMS voltage and current and include the power factor for real power. Resistors also have a power rating: pick one rated for at least twice the watts the circuit puts through it.