LED Series Resistor Calculator

About this calculator

An LED needs a series resistor to limit current to a safe level, since LEDs have almost no inherent resistance and will draw excessive current (and burn out) if connected directly across a supply. This calculator applies Ohm's law directly: R = (V_supply − V_forward) ÷ I_target, where V_forward is the LED's forward voltage drop (roughly 1.8-2.2V for red, 3.0-3.4V for blue or white) and I_target is the desired current, commonly 20 mA for a standard 5mm indicator LED.

Hobbyist electronics builders, students learning basic circuit design, and Arduino or microcontroller project makers use this calculation on nearly every project involving an indicator light, since picking too small a resistor shortens the LED's life or destroys it instantly, while too large a resistor makes the LED noticeably dim.