Ohm's Law Calculator
Find voltage, current, resistance and power from any two
All Four Values
- 1Pick the two you knowAny pair of V, I, R and P
- 2Enter their valuesIn volts, amps, ohms or watts
- 3Get the restAll four values at once
Ohm's law and power
Ohm's law relates voltage, current and resistance; the power formula adds watts. From any two you can find the other two.
V = I × R · I = V ÷ R · R = V ÷ I
P = V × I = I² × R = V² ÷ R
A quick example
A 12 V supply across a 6 Ω resistor drives 2 A, and the resistor dissipates 24 W. For a 60 W bulb at 230 V, the current is about 0.26 A and its working resistance about 882 Ω.
Real-world limits
Ohm's law applies directly to resistive loads. For AC motors and electronics, use apparent power and power factor instead (see our Watts to Amps and kVA to kW calculators).
Related calculators
- Watts to Amps CalculatorConvert power to current (and back) for AC and DC
- Resistor Color Code CalculatorRead a resistor's value from its colour bands
- Wire Size CalculatorCable size in sq mm for your load, length and voltage drop
- kVA to kW CalculatorConvert apparent power to real power, and size a generator or inverter
Frequently asked questions
What is Ohm's law?
The current through a conductor is proportional to the voltage across it: V = I × R, where R is the resistance in ohms.
How do I find resistance from power and voltage?
R = V² ÷ P. A 1,000 W heater on 230 V has a resistance of about 52.9 Ω.
Does Ohm's law work for LEDs?
Not directly: LEDs aren't resistors. You use Ohm's law to size the series resistor: R = (supply − LED voltage) ÷ desired current.
Last reviewed: October 2026