How it works
Ohm's law says that the current through a conductor is proportional to the voltage across it: V = I × R. Power follows from P = V × I, and by putting Ohm's law into that formula you also get P = I²R and P = V²/R.
With any two of the four quantities known, the other two are fixed. For example, a 230 V supply across a 46 Ω heater element gives I = 230 ÷ 46 = 5 A and P = 230 × 5 = 1150 W.
Use volts, amperes, ohms and watts in the boxes. For milliamperes divide by 1000 first (20 mA is 0.02 A), and for kilo-ohms multiply by 1000 (4.7 kΩ is 4700 Ω).
Formulas used
- V = I × R
- P = V × I
- P = I² × R
- P = V² ÷ R
- I = V ÷ R
- R = V ÷ I
Worked example
230 V across 46 Ω: current = 230 ÷ 46 = 5 A, power = 230 × 5 = 1150 W.
The calculator opens with these example values, so you can compare its answer with the text above.
Frequently asked questions
What is Ohm's law?
Ohm's law states that V = I × R: the voltage across a resistor equals the current through it multiplied by its resistance.
Does Ohm's law work for AC?
It works for purely resistive loads such as heaters and incandescent lamps, using RMS values. For coils and capacitors the resistance is replaced by impedance.
How do I find power if I know voltage and resistance?
Use P = V² ÷ R. A 12 V supply across 6 Ω gives 144 ÷ 6 = 24 W.
Why must I leave two boxes empty?
Two known values fix the other two. If you give three or four values they could disagree, so the calculator asks for exactly two.