LED Resistor Calculator
Enter your supply voltage, the LED's forward voltage, and your target current to find the series resistor needed.
How the LED series resistor is calculated
An LED needs a series resistor to limit current to a safe level, since LEDs don't have enough internal resistance on their own. The resistor value is R = (Vsupply − Vforward) / I, where Vforward is the LED's voltage drop (found on its datasheet) and I is your target current in amps.
The resistor also dissipates power as heat, calculated as P = I² × R. Choosing a resistor rated for at least this power (with some margin) prevents it from overheating.
Frequently asked questions
What's a typical LED forward voltage?
It varies by LED color and type: roughly 1.8-2.2 V for red, 2.0-2.4 V for yellow/orange, and 2.8-3.4 V for green, blue, or white LEDs — always check the specific LED's datasheet for an accurate figure.
What's a safe current for a standard LED?
20 mA is a common safe operating current for standard 5mm indicator LEDs, though this varies by LED — check the datasheet's maximum forward current rating and stay comfortably under it.
What happens if I skip the resistor?
Without a current-limiting resistor, an LED connected directly to a voltage source higher than its forward voltage will draw excessive current, overheat, and likely burn out almost immediately.