Calculate LED resistor

Choose a series resistor estimate and wattage for one or more LEDs.

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ToolLED Resistor Calculator
Input
Output
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Preview

How it works

Resistance is (supply − summed forward voltage) ÷ desired current, and resistor power is current² × resistance. Series LEDs add forward voltages before subtraction.

  • Forward voltage and current are explicit part inputs.
  • The result is an ideal DC estimate.

Worked example

Red LED on a 9V supply
9V supply, 2V forward voltage, 20mA target current
Input
											Supply voltage volts: 9
Led forward voltage volts: 2
Led forward current milliamps: 20
Output
												Resistance ohms: 350
Nearest e12 ohms: 390
Actual current milliamps: 17.9487
Power dissipated watts: 0.1256
Recommended power rating watts: 0.5

When to use this

Hobby circuits limit indicator current, design worksheets check wattage, and battery projects estimate LEDs in series.

Edge cases

  • A supply below summed forward voltages leaves no positive resistor voltage.
  • Forward voltage varies with current and temperature.
  • A resistor rated exactly at calculated power has no thermal margin.