Ohm's law and power
Ohm's law says the current through a resistor is proportional to the voltage across it: V = I × R. Combined with the power formula P = V × I, any two of the four quantities give you the other two.
| You know | Formulas |
|---|---|
| V and I | R = V ÷ I, P = V × I |
| V and R | I = V ÷ R, P = V² ÷ R |
| I and R | V = I × R, P = I² × R |
| V and P | I = P ÷ V, R = V² ÷ P |
| I and P | V = P ÷ I, R = P ÷ I² |
| R and P | V = √(P × R), I = √(P ÷ R) |
Worked examples
- A 12 V supply driving 0.5 A means a 24 Ω load dissipating 6 W.
- A 0.25 W resistor of 100 Ω can take at most √(0.25 × 100) = 5 V, or 50 mA.
- A 2,000 W heater on 230 V draws about 8.7 A, so it has a resistance of 26.45 Ω when hot.
Use base units: volts, amps, ohms and watts. Enter 4.7 kΩ as 4700 and 20 mA as 0.02.
How this works
The page uses V = I × R and P = V × I, rearranged for the pair you chose, as in the table above. It shows the first unknown as the headline and the second with the two inputs. If a formula would divide by zero, the page reports it rather than showing infinity. Everything runs in your browser.
Frequently asked
What is Ohm's law?
V = I × R: voltage equals current times resistance.
How do I calculate power from voltage and resistance?
P = V² ÷ R. 5 V across 100 Ω is 0.25 W.
Does Ohm's law work for AC?
For resistive loads with RMS values, yes. Motors and other reactive loads need impedance and power factor.
How do I enter kilohms or milliamps?
Convert to base units: 4.7 kΩ = 4700, 20 mA = 0.02.
Sources
Reviewed Oct 17, 2026 · test vectors fixtures/calculators/electronics/ohms-law-calculator/