How an LED Resistor Calculator Works
Light-emitting diodes are current-driven devices. Connect one directly to a voltage source and it will draw far more current than it can handle, destroying itself in an instant. A series resistor limits that current to a safe level, and this calculator works out exactly what value you need.
The math is a direct application of Ohm's law. The resistor has to drop the difference between your supply voltage and the LED's forward voltage, while passing the desired current. Once you know the exact value, you round to the nearest standard resistor and check the power rating.
The LED Resistor Formula
The resistor value and the power it must dissipate are found with these two equations, where current is expressed in amps:
- Resistance R = (V_supply − V_LED) / I
- Power P = (V_supply − V_LED) × I
- Current in amps = Current in mA ÷ 1000
Tips for Driving LEDs Safely
Getting the resistor right keeps your LED bright and long-lived. A few practical guidelines cover most hobby and prototyping situations.
- Check the LED datasheet for its forward voltage and maximum continuous current rather than guessing.
- When the exact value falls between standard resistors, choose the next higher value to stay on the safe side.
- Use a resistor rated for at least double the calculated power dissipation to avoid heat problems.
- For multiple LEDs, give each its own resistor rather than sharing one, since LEDs do not divide current evenly.
Frequently Asked Questions
What current should I use for an LED?
Most standard 5 mm LEDs are rated for a maximum of around 20 mA, which is why it is a common default. You can run them at lower currents like 10 mA for a dimmer but more efficient output. Always confirm the maximum current in the LED datasheet before choosing.
What is the forward voltage of an LED?
Forward voltage is the voltage the LED drops when conducting, and it depends on the color. Red and yellow LEDs are typically around 1.8–2.2 V, green around 2.2–2.6 V, and blue and white LEDs around 3.0–3.4 V. This value is subtracted from the supply voltage in the calculation.
What power rating resistor do I need?
For typical small LEDs at 20 mA, the resistor dissipates only a fraction of a watt, so a standard 1/4 W resistor is usually fine. At higher currents or larger voltage drops, step up to a 1/2 W or 1 W resistor. This calculator recommends a rating based on the calculated power.