Two known values give the other two. Fill in any two fields below and leave the others empty.
Enter two values.
Check the resistor power rating. A common small resistor is rated about a quarter watt.
How it is calculated
V = I × R. P = V × I. Combining them: P = I² × R and P = V² ÷ R. If you give more than two values, the first pair in the order voltage, current, resistance is used.
A worked example
A 5 V supply across a 330 ohm resistor gives I = 5 ÷ 330 = 15.2 mA and P = 5 × 0.0152 = 0.076 W, well under a quarter watt.
Common mistakes
- Mixing milliamps and amps. Enter current in amps (10 mA = 0.01 A).
- Forgetting the power rating of the resistor.
- Applying the law to parts that are not resistors, such as LEDs, which need a series resistor.
Related
LED resistor and voltage divider calculator, battery runtime calculator.
Last reviewed: November 2026. The calculator runs in your browser and sends nothing anywhere. Results are ideal-case figures; add margin before you order parts.