Ohm's Law Calculator
Leave exactly one field blank and the calculator solves it from the other two — and shows power as a bonus. The classic V = I × R with all its rearrangements.
Open calculatorLast updated: 2026-09-15
How the calculation works
- Exactly one field must be blank — that is the unknown the calculator solves for.
- The tool applies the corresponding rearrangement of Ohm's law and reports all four quantities.
- Power is computed from P = V × I, which is equivalent to I²R and V²/R.
Worked example
Interpreting the result
Current rises proportionally with voltage and inversely with resistance. If the computed current exceeds the rating of the wiring or device, something must give — a breaker trips, a fuse blows or a conductor overheats. Power tells you the heat or work rate: in resistive loads it all becomes heat.
Assumptions
- DC circuit or purely resistive AC load (heaters, incandescent lamps).
- Fixed resistance — it does not model how resistance changes with temperature.
Limitations
- Reactive AC loads (motors, transformers, electronics) need impedance and power factor, not plain resistance.
- Non-linear devices (diodes, LED drivers) do not obey Ohm's law in this simple form.
Current and power for common voltages and resistances
Each row is the calculator's result with voltage and resistance entered.
| Voltage | Resistance | Current | Power dissipated |
|---|---|---|---|
| 5 V | 10 Ω | 500 mA | 2.5 W |
| 5 V | 100 Ω | 50 mA | 0.25 W |
| 5 V | 1 kΩ | 5 mA | 0.025 W |
| 12 V | 10 Ω | 1.2 A | 14.4 W |
| 12 V | 100 Ω | 120 mA | 1.44 W |
| 12 V | 1 kΩ | 12 mA | 0.144 W |
| 24 V | 10 Ω | 2.4 A | 57.6 W |
| 24 V | 100 Ω | 240 mA | 5.76 W |
| 24 V | 1 kΩ | 24 mA | 0.576 W |
| 120 V | 10 Ω | 12 A | 1440 W |
| 120 V | 100 Ω | 1.2 A | 144 W |
| 120 V | 1 kΩ | 120 mA | 14.4 W |
| 230 V | 10 Ω | 23 A | 5290 W |
| 230 V | 100 Ω | 2.3 A | 529 W |
| 230 V | 1 kΩ | 230 mA | 52.9 W |
I = V ÷ R and P = V² ÷ R.
The six formulas behind the triangle
- V = I × R: voltage across a resistance.
- I = V ÷ R: current for a given voltage.
- R = V ÷ I: resistance from a measured voltage and current.
- P = V × I: power from voltage and current.
- P = I² × R: power when you know the current, which is why heating rises with the square of current.
- P = V² ÷ R: power when you know the voltage.
Worked scenario: choosing the power rating of a resistor
Put 12 V across a 100 Ω resistor and the calculator shows 120 mA flowing and 1.44 W dissipated. A ¼ W resistor would be far over its rating. A common rule is to choose a power rating at least twice the calculated dissipation, so here a 3 W part or larger.
Mistakes to avoid
- Unit slips: milliamps entered as amps, or kilohms as ohms, change the answer by a factor of 1,000.
- Applying Ohm's law to parts that are not simple resistances, such as LEDs, diodes and lamps whose resistance changes with temperature.
- Using it for AC circuits with inductors or capacitors, where reactance and phase matter as well.
Frequently asked questions
How do I calculate power from voltage and resistance?
Use P = V² / R. For example, 120 V across a 24 Ω element gives 120² / 24 = 600 W.
Does Ohm's law work for AC?
Only for purely resistive AC loads. With inductive or capacitive loads, replace R with impedance Z and account for power factor: P = V × I × PF.