LED Resistor Calculator

Calculate the correct series resistor for an LED using supply voltage, LED forward voltage, and desired LED current.

LED Resistor Inputs

V
V
mA
Enter the LED's typical forward voltage from its datasheet for the most accurate result.

Advanced Options

Used to recommend the nearest standard resistor value.
Required Resistor
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Resistor Results

Required Resistance
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Standard Resistance
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Resistor Tolerance
±5%
Actual LED Current
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Resistor Voltage
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Resistor Power
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LED Circuit Results

LED Power
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Total Circuit Power
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LED Efficiency
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Recommended Minimum Wattage
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Circuit Current
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Calculation

Step 1: Calculate Voltage Across Resistor
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Step 2: Calculate Resistance
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Step 3: Calculate Resistor Power
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LED Resistor Formula

R=(VS− VF) / I

Where:

  • R=required resistor resistance in ohms
  • VS=supply voltage
  • VF=LED forward voltage
  • I=desired LED current in amperes
P=I² × R

The resistor power rating should be higher than the calculated resistor power to provide an appropriate safety margin.

Example: 5V LED Resistor

Suppose a red LED has a forward voltage of 2 V and is operated from a 5 V supply at 20 mA.

R=(5 − 2) / 0.020
R=150 Ω

Therefore, a 150 Ωresistor is required for approximately 20 mA under the assumed forward voltage.

The resistor power is approximately 0.06 W, so a resistor with a rating comfortably above this value should be selected.

Important Notes

  • LED forward voltage varies with current, temperature, and LED type.
  • Use the forward-voltage value from the LED manufacturer's datasheet for accurate circuit design.
  • The calculated resistance is based on the assumed forward voltage remaining constant.
  • Never connect a typical LED directly to a voltage source without current limiting.
  • For high-power LEDs, a constant-current driver may be preferable to a simple series resistor.