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LED Resistor Calculator

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Every LED needs a current-limiting resistor to prevent it from drawing too much current and burning out. This calculator determines the exact resistor value, suggests the nearest standard E12 series resistor, and shows the power dissipation and actual current with the standard value.

R = (Vsupply − Vforward) ÷ Iforward
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How to use this calculator

Step-by-Step Instructions

1. **Enter your supply voltage** This is the voltage source powering your circuit (e.g., 5V from USB, 3.3V from microcontroller, 9V battery, 12V adapter).
2. **Enter the LED forward voltage (Vf)** Check your LED datasheet. Typical values: Red=1.8-2.2V, Yellow/Green=2.0-2.4V, Blue/White=3.0-3.6V, Infrared=1.2-1.5V.
3. **Enter the desired forward current** Standard LEDs typically run at 20mA. High-brightness LEDs may use 30-350mA. Always check the datasheet for maximum rating.
4. **Click Calculate** The calculator shows the exact calculated value, nearest standard resistor (E12 series), power dissipation of the resistor, and actual LED current with the standard value.

Worked Example

You want to connect a **red LED (Vf=2.0V, If=20mA)** to a **5V Arduino pin**:

R = (5V − 2.0V) ÷ 0.020A = 3.0V ÷ 0.020A = **150Ω**

Nearest E12 standard: **150Ω**(exact match). Power: 0.06W a 1/8W resistor works fine.

Pro Tips

  • Always choose the next*higher*standard resistor value, never lower. A slightly higher resistance means slightly less current, which is safer for the LED.
  • For the resistor power rating, use at least 2× the calculated power dissipation for reliability. If the calculation shows 0.06W, use at least a 1/8W (0.125W) resistor.
  • When driving multiple LEDs in series, add up their forward voltages: Vf_total = Vf1 + Vf2 + Vf3, then use R = (Vs − Vf_total) ÷ If.