LED Resistor Calculator

Pick a current-limiting resistor so an LED runs safely.

How the Math Works

The LED Resistor Calculator uses Ohm's Law to determine the correct series resistor value for an LED circuit. The formula R = (Vsupply - Vled) / I works by first finding the voltage difference between your power source and the LED's forward voltage drop, then dividing by the desired LED current. For example, with a 9V battery, LED forward voltage of 2V, and desired current of 20mA, the calculation becomes (9-2)/0.02 = 350 ohms. This ensures the LED receives exactly the right amount of current to operate brightly without burning out.

Practical Applications

To use this calculator practically, first identify your power supply voltage (Vsupply), find the LED's forward voltage (Vled) from its datasheet, and determine the recommended operating current (I) also listed in the datasheet. Enter these three values into the calculator to get the exact resistor value needed. When building your circuit, choose a resistor with at least 25% more power rating than calculated (for instance, if your calculation shows 1/4 watt, use a 1/3 watt resistor) to ensure reliable operation and prevent overheating.

Day-to-Day Use

This calculator empowers anyone to safely integrate LEDs into everyday projects, from repair work to creative builds. Whether you're installing LED turn signals on a bike, creating ambient lighting for a room, or troubleshooting a broken appliance with LED indicators, knowing the right resistor prevents premature LED failure and ensures optimal brightness. It transforms potentially frustrating trial-and-error shopping for resistors into confident, precise electronics work that anyone can master.

Worked example

5 V, 2 V LED, 20 mA → 150 Ω.

FAQ