Ohm's Law Calculator
Solve Ohm's law: enter voltage and current to get resistance in ohms and power in watts, using R = V ÷ I and P = V × I.
How this calculator works
Ohm’s law is the starting relationship for almost every circuit: it ties together voltage, current, and resistance. Give it any two of those and it fills in the rest, along with the power the circuit consumes.
This tool takes voltage and current, then returns the resistance and the power so you can size components or check a circuit’s behavior at a glance.
Formula: Resistance = Voltage ÷ Current; Power = Voltage × Current.
Worked example
A circuit at 12 V carrying 3 A:
- Resistance = 12 ÷ 3 = 4 Ω
- Power = 12 × 3 = 36 W
At 12 volts
| Current | Resistance | Power |
|---|---|---|
| 1 A | 12 Ω | 12 W |
| 2 A | 6 Ω | 24 W |
| 3 A | 4 Ω | 36 W |
| 4 A | 3 Ω | 48 W |
How to use
Enter the voltage in volts and the current in amps for the circuit you are checking. The calculator returns resistance in ohms and power in watts from the same two values. Keep every input in base units — volts and amps, not millivolts or milliamps — so the results stay accurate.
Electrical work is code-governed and safety-critical. Confirm circuit, breaker, and conductor sizing with a licensed electrician and your local code.
Frequently asked questions
What is Ohm's law?
Ohm's law links voltage, current, and resistance: resistance equals voltage divided by current. From the same two inputs you can also find power, which is voltage times current. With 12 V and 3 A, the resistance is 12 ÷ 3 = 4 ohms and the power is 12 × 3 = 36 W. It is the core relationship behind almost every basic circuit calculation.
How do I calculate resistance from voltage and current?
Divide the voltage by the current. For a circuit at 12 V carrying 3 A, the resistance is 12 ÷ 3, which equals 4 ohms. A higher current for the same voltage means a lower resistance, since current sits in the denominator. This calculator does that division for you and also returns the power the circuit is using.
How do I find power with Ohm's law?
Multiply voltage by current to get power in watts. At 12 V and 3 A, the power is 12 × 3 = 36 W. Power tells you how much energy the circuit turns into heat, light, or motion each second, which is what you use to size resistors, fuses, and heat sinks. This tool reports power alongside the resistance from the same inputs.
What units does this calculator use?
Voltage is entered in volts, current in amps, and the tool returns resistance in ohms and power in watts. In the worked example, 12 volts and 3 amps give 4 ohms and 36 watts. Keep every input in these base units — do not mix in millivolts or milliamps — so the resistance and power results come out correctly.
Estimates only. Verify quantities with your supplier before purchasing.