Ohm's Law is the fundamental relationship between voltage, current, and resistance in an electrical circuit. This calculator takes voltage and current and returns resistance and power — the two quantities most often needed when sizing components or checking a circuit design.
The formulas
For a 12 V supply drawing 2 A: resistance is 12 ÷ 2 = 6 Ω, and power is 12 × 2 = 24 W.
Worked examples
| Voltage | Current | Resistance | Power |
|---|---|---|---|
| 9 V | 0.5 A | 18 Ω | 4.5 W |
| 230 V | 4 A | 57.5 Ω | 920 W |
| 5 V | 0.1 A | 50 Ω | 0.5 W |
The Ohm's Law triangle
A common way to remember all three rearrangements is the V-I-R triangle: cover the quantity you want to find, and the remaining two show the operation. Covering V shows I × R; covering I shows V ÷ R; covering R shows V ÷ I.
Common mistakes
- Mixing units. Voltage must be in volts and current in amps for the result to be in ohms directly — using milliamps without converting first gives a result off by a factor of 1,000.
- Forgetting AC power factor. This calculator assumes a simple resistive DC circuit; real AC circuits with reactive loads need the power factor included for accurate power figures.
Tips
- Double-check a calculated resistance against the component's rated tolerance — real resistors vary by a percentage from their labeled value.
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that voltage equals current multiplied by resistance (V = I × R), describing the relationship between these three quantities in an electrical circuit.
How do I find resistance from voltage and current?
Divide voltage by current: R = V ÷ I. For example, 12 V divided by 2 A equals 6 Ω.
Does this calculator work for AC circuits?
It applies the basic resistive relationship suitable for DC circuits and simple resistive AC loads. Circuits with significant reactance (inductors or capacitors) need impedance calculations instead of simple resistance.