Ohm's law calculator
Enter any two values and the other two are calculated instantly. The two fields you changed last are kept; the rest are worked out (shaded).
Ohm's law formulas
Ohm's law says the current through a resistor is proportional to the voltage across it: V = I × R. Combined with the power equation P = V × I, any two quantities give the other two:
| Find | From V and I | From V and R | From I and R | From P and… |
|---|---|---|---|---|
| Voltage V | — | — | I × R | P ÷ I, √(P × R) |
| Current I | — | V ÷ R | — | P ÷ V, √(P ÷ R) |
| Resistance R | V ÷ I | — | — | V² ÷ P, P ÷ I² |
| Power P | V × I | V² ÷ R | I² × R | — |
Worked examples
| Circuit | Known | Result |
|---|---|---|
| 5 V across a 220 Ω resistor | 5 V, 220 Ω | 5 V, 22.73 mA, 220 Ω, 113.6 mW |
| 20 mA through 150 Ω | 20 mA, 150 Ω | 3 V, 20 mA, 150 Ω, 60 mW |
| 12 V, 0.5 A load | 12 V, 500 mA | 12 V, 500 mA, 24 Ω, 6 W |
| A 1/4 W, 1 kΩ resistor at full power | 250 mW, 1 kΩ | 15.81 V, 15.81 mA, 1 kΩ, 250 mW |
The last row shows the most useful check in practice: a 1/4 W 1 kΩ resistor can only take 15.8 V before it reaches its rating. Keep at least a 2× margin — see resistor power ratings. For LEDs, use the dedicated LED resistor calculator.
FAQ
What is Ohm's law?
Voltage equals current times resistance (V = I × R). It holds for resistors and most conductors at a constant temperature.
How do I calculate the power of a resistor?
P = V × I = I² × R = V² ÷ R. Choose a resistor rated at least twice the calculated power.
How do I enter kilohms?
Type 4.7k, 4k7 or 4700 in the resistance field — all mean 4.7 kΩ.