LED Resistor Sizing Formula and Calculations [With Calculator]

Light emitting diodes are very popular electronic components that are found everywhere around us. The proper operation of LED requires a resistor which controls its operating voltage and current characteristics.

LED Resistor Sizing Formula

R =
Vin - VF / lF


Vin = Supply voltage

VF = Forward voltage of LED

IF = Forward current of LED

R = Resistor

Circuit diagram

led resistor circuit diagram

LED Resistor Sizing calculator

IF (mA)


Example 1: A bright red 5mm led has a forward voltage drop of 2 V. The forward current is 20 mA. Whereas the input source is a 12 V battery. Size the resistor.

Solution: R = {Vin – VF}/IF

R = {12 V – 2 V} / 20 mA = 500 Ω

Example 2: The 5mm green LED has the forward current of 10 mA. The voltage drop across the LED is 3.4 V. Select the appropriate resistor that would allow the safe operation. The input is 10 volts.

Solution: R = {Vin – VF}/IF

R = { 10 –  3.4}/10 mA = 660 Ω

Important concepts you should remember

Polarity of LED

Just like normal rectifier diode, the LED (light emitting diode) has anode and cathode. It should be forward biased for the proper working. The anode (+ve terminal) is comparatively longer in length and one can easily distinguish it from the cathode (-ve).

led symbol and anode cathode

Forward voltage (VF)

The forward voltage is the nominal voltage which is dropped across the proper biased LED and is necessary for the proper operation of LED. The forward voltage of LED ranges between 2-5 V. You should always refer to the datasheet to find the exact value.

Forward current (VF)

The current which is produced by LED during its forward operation. Usually this value if between 10 – 100 mA.

The LED data sheet looks like this:


Other formulas:

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