Ohm's Law Calculator
Enter any two of voltage (V), current (A), resistance (Ω) and power (W) to solve the other two with V = IR and P = VI, shown with SI prefixes. How it works
Use the Ohm's Law Calculator
About the Ohm's Law Calculator
With 12 V across a 6 Ω resistor the current is 12 ÷ 6 = 2 A and the power is 12 × 2 = 24 W. Enter any two of voltage, current, resistance and power and the calculator solves the other two with V = I × R and P = V × I: 12 V and 2 A, 12 V and 6 Ω, or 6 Ω and 24 W all give this same circuit.
The same 12 V, 2 A, 6 Ω, 24 W circuit from each pair of inputs
| You enter | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| Voltage and current (12 V, 2 A) | 12.000 V | 2.000 A | 6.000 Ω | 24.000 W |
| Voltage and resistance (12 V, 6 Ω) | 12.000 V | 2.000 A | 6.000 Ω | 24.000 W |
| Voltage and power (12 V, 24 W) | 12.000 V | 2.000 A | 6.000 Ω | 24.000 W |
| Current and resistance (2 A, 6 Ω) | 12.000 V | 2.000 A | 6.000 Ω | 24.000 W |
| Current and power (2 A, 24 W) | 12.000 V | 2.000 A | 6.000 Ω | 24.000 W |
| Resistance and power (6 Ω, 24 W) | 12.000 V | 2.000 A | 6.000 Ω | 24.000 W |
Resistance and power gives the positive roots only: V = √(P × R) and I = √(P ÷ R).
More solved examples
| You enter | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| 5 V and 0.02 A | 5.000 V | 20.000 mA | 250.000 Ω | 100.000 mW |
| 3.3 V and 10000 Ω | 3.300 V | 330.000 μA | 10.00 kΩ | 1.089 mW |
| 230 V and 2000 W | 230.000 V | 8.696 A | 26.450 Ω | 2.00 kW |
Values as the tool prints them. The fields take plain volts, amps, ohms and watts, so 20 mA is typed as 0.02 and 10 kΩ as 10000; only the results use prefixes.
How to use this tool
- Type any two of voltage, current, resistance and power. Your entries stay as typed.
- Read the other two values in the result, shown with SI prefixes such as kΩ, mA and μA.
- If you typed a third value that disagrees, the page says which one and which two fields the results use.
Frequently asked questions
- How do I calculate current from voltage and resistance?
- Divide the voltage by the resistance: I = V ÷ R. With 3.3 V across 10000 Ω the current is 330.000 μA and the power is 1.089 mW.
- How do I find resistance from power and voltage?
- R = V² ÷ P. A 2000 W heater on 230 V has 230² ÷ 2000 = 26.450 Ω and draws 8.696 A.
- What is the formula for power in a resistor?
- P = V × I, which also equals I² × R and V² ÷ R. Across 5 V with 20 mA the power is 5 × 0.02 = 100.000 mW and the resistance is 250.000 Ω.
- What happens if I fill in three or four fields?
- The calculator solves from the first pair it finds in the order voltage and current, voltage and resistance, voltage and power, current and resistance, current and power, then resistance and power. Any other value you typed is compared with the solution, and one that differs by more than 0.5% is named on the page. With 12 V, 2 A and 7 Ω, the page says the resistance does not match and that the results use voltage and current, so the resistance shown is 6.000 Ω.
- Can resistance or power be negative?
- No. A negative resistance or power is refused with a message. Voltage and current can be negative, but not with opposite signs together, because the resistance and power of a resistor would then be negative and that pair is refused. A zero that would be a divisor, such as zero current when finding resistance, also gets a message instead of a result.
- Does it tell me whether a resistor can handle the power?
- No. It computes the power but has no component ratings, so it cannot judge a part. Compare the power with the rating in the datasheet.
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