LED Resistor Calculator
Every LED needs a current-limiting resistor to avoid burning out. This calculator tells you exactly what resistor to use based on your supply voltage, LED type, and desired brightness. It also rounds up to the nearest standard E24 value and checks the power rating so your resistor does not overheat.
The Formula
R = (V_supply - V_LED) / I_LED
The resistor drops the excess voltage so the LED receives its rated forward voltage at the correct current. Without it, the LED draws too much current and burns out almost instantly.
Common LED Forward Voltages
| LED Color | Forward Voltage | Typical Current |
|---|---|---|
| Red | 1.8 - 2.2V | 20 mA |
| Orange/Yellow | 2.0 - 2.1V | 20 mA |
| Green | 2.0 - 3.0V | 20 mA |
| Blue/White | 3.0 - 3.4V | 20 mA |
| Infrared | 1.2 - 1.6V | 20 - 50 mA |
Worked Example
Running a red LED (2.0V, 20 mA) from a 5V Arduino pin:
- R = (5V - 2V) / 0.020A = 150 ohms exactly
- Nearest standard value: 150 ohms (it's in the E24 series)
- Power dissipated: 3V x 0.02A = 60 mW (a 1/8W resistor is fine)
Multiple LEDs in Series
When wiring LEDs in series, their forward voltages add up. Three red LEDs in series need 6.0V, so a 5V supply is not enough. You would need at least 7V or more. This calculator handles that math automatically.