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LED Resistor Calculator (R=(Vs-Vf)/If)

Quick answer

This calculator finds the series resistor for any LED with R = (Vs - Vf)/If.

  • Enter supply voltage, forward voltage, and current for the standard value and power rating.

Figures are estimates from this calculator’s standard formula — adjust the inputs in the tool below for your exact number.

How the LED Resistor Calculator Works

R = (Vs - Vf) / If

Calculates the resistor needed to limit current through an LED. Resistance equals the voltage difference (supply minus LED forward voltage) divided by desired current. Worked example: a red LED (Vf = 2 V, If = 20 mA) on 12 V needs R = (12-2)/0.02 = 500 ohm; the nearest standard value 470 ohm gives 21.3 mA and burns 0.213 W, so use a 1/4 W or larger resistor. For multiple identical LEDs in series, sum the forward voltages first: two red LEDs on 12 V need R = (12-4)/0.02 = 400 ohm, and the same 1/4 W resistor still suffices.

  1. 1

    Step 1

    Enter the supply voltage (battery or power supply)

  2. 2

    Step 2

    Enter LED forward voltage (varies by color)

  3. 3

    Step 3

    Enter desired current (typically 20mA)

  4. 4

    Step 4

    Specify number of LEDs in series

Use Cases

Protecting LEDs in electronics projects

Designing LED lighting panels

Arduino and Raspberry Pi projects

Car dashboard and indicator lights

Tips

  • 1

    LED forward voltage: Red 1.8-2V, Green/Yellow 2-2.2V, Blue/White 3-3.4V

  • 2

    Use 10-20mA for normal brightness

  • 3

    Choose resistor power rating 50% higher than calculated

  • 4

    For parallel LEDs, use one resistor per LED

Common Mistakes

  • Forgetting to add a resistor, burning the LED instantly

  • Using wrong LED voltage (varies by color)

  • Connecting parallel LEDs with a single resistor

  • Not accounting for resistor power rating (Watts)

FAQs

Why does LED voltage vary by color?

Forward voltage depends on semiconductor material. Blue and white LEDs use materials with higher bandgap energy, requiring higher voltage.

What happens if I use a smaller resistor than needed?

Higher current flows through the LED, reducing its lifespan or burning it instantly. Always use equal or higher resistance.

Can I connect multiple LEDs in series?

Yes, but ensure total LED voltages are at least 2-3V less than supply voltage for the resistor to work properly.

What resistor do I need for a 12 V LED?

For a typical white LED (Vf = 3.2 V, 20 mA): R = (12-3.2)/0.02 = 440 ohm, so use 470 ohm. For a red LED (Vf = 2 V): R = 500 ohm, use 470 or 560 ohm standard values.

How do I pick the resistor power rating?

Power P = If2 x R. At 20 mA through 470 ohm, P = 0.188 W: choose a 1/4 W resistor minimum. Rule of thumb: pick a rating at least 2x the calculated dissipation.

Can I run an LED without a resistor?

Only with a constant-current driver. Connecting an LED straight to a voltage source causes thermal runaway - current rises with temperature until the junction burns out.

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