How to Use Ohm's Law — Voltage, Current & Resistance
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. If you know any two of the three quantities, you can calculate the third. The law also extends to electrical power — watts consumed by a component can be calculated from any two of the three variables. Ohm's Law is the foundation of electronics and circuit analysis.
Last updated: March 31, 2026
The Formula
V = I × R (Voltage = Current × Resistance) I = V / R (Current = Voltage / Resistance) R = V / I (Resistance = Voltage / Current) P = V × I (Power = Voltage × Current) P = I² × R (Power from current and resistance) P = V² / R (Power from voltage and resistance)
Variable Definitions
| Symbol | Name | Description |
|---|---|---|
| V | Voltage | Electrical potential difference (pressure) that drives current through a circuit, measured in volts |
| I | Current | The flow rate of electric charge through the circuit, measured in amperes (amps) |
| R | Resistance | Opposition to current flow, measured in ohms (Ω) |
| P | Power | Rate of energy consumption or generation, measured in watts (W) |
Step-by-Step Example
A 12V car battery powers a headlight bulb with a resistance of 4Ω. Find the current and power consumed.
Given
Solution
- 1Calculate current using I = V / R:
I = 12 / 4 = 3 A - 2Calculate power using P = V × I:
P = 12 × 3 = 36 W - 3Verify with P = I² × R:
P = 3² × 4 = 9 × 4 = 36 W ✓
Current = 3 A, Power = 36 W. The headlight draws 3 amps and consumes 36 watts.
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Common Mistakes to Avoid
Confusing voltage with current — voltage is pressure (force), current is flow (rate). Both are needed; neither alone causes work.
Using milliamps without converting — if current is 200 mA, convert to 0.2 A before using the formula.
Applying Ohm's Law to non-ohmic components — diodes, LEDs, and capacitors do not follow Ohm's Law linearly.
Forgetting that resistance changes with temperature — a wire's resistance increases as it heats up, affecting high-current applications.