Engineering Tools
LED Resistor Calculator
Find the perfect series resistor for your LED to ensure it lights up brightly without burning out.
The LED Resistor Formula (Ohm's Law)
To calculate the correct series resistor for an LED, you use a variation of Ohm's Law: R = (Vs - Vf) / I
- Vs (Source Voltage): The voltage of your power supply (e.g., a 5V USB wall adapter, or a 9V battery).
- Vf (Forward Voltage): The voltage drop across the LED. This energy is consumed by the LED to produce light.
- I (Forward Current): The amount of current flowing through the LED. Most standard 5mm LEDs run optimally between 15mA and 20mA (0.015A to 0.020A).
Common LED Forward Voltages (Vf)
Different colors of LEDs require different amounts of voltage to turn on due to the semiconductor materials used to create them.
| LED Color | Typical Forward Voltage (Vf) |
|---|---|
| Infrared (IR) | 1.2V - 1.5V |
| Red | 1.8V - 2.2V |
| Yellow / Orange | 2.0V - 2.2V |
| Green | 2.2V - 3.0V |
| Blue / White / UV | 3.0V - 3.4V |
Why Resistor Wattage Matters
When dropping voltage across a resistor to protect an LED, that excess energy is dissipated as heat. The power dissipation formula is P = Voltage Drop × Current.
If your power supply is 12V and your LED is 2V, the resistor must drop 10V. At 20mA, that is 10V × 0.02A = 0.2 Watts. A standard 1/8W (0.125W) resistor would overheat and burn up in seconds. In this scenario, you must use a 1/4W or 1/2W resistor.
Our calculator automatically checks the power dissipation and recommends the safest minimum wattage rating for your circuit.