LED Resistor Calculator

Calculate the proper current-limiting resistor value for your LED circuit. This tool processes all data locally in your browser. No information is ever sent to any server. Completely free, no registration required.

How to Use the LED Resistor Calculator

1. Enter your input values above
2. Results update automatically
3. Copy or download the output

What is a LED Resistor Calculator?

An LED Resistor Calculator computes the correct resistor value to safely power LEDs at their rated current without burning them out. LEDs are current-driven devices — apply too much current and they instantly fail. A resistor in series with the LED limits current to the designed value. This calculator handles single LEDs, LEDs in series, and LEDs in parallel, with instant results showing the resistor value (ohms), power rating (watts), and the standard resistor color code.

How Does It Work?

Enter supply voltage, LED forward voltage (from datasheet, typically 2-3.3V depending on color), and desired LED current (typically 20mA for standard LEDs). The calculator applies Ohm's Law: R = (Vs − Vf) ÷ I. The result shows the exact resistance, the next standard resistor value (E12 or E24 series), power dissipation (P = I²R), and the resistor color bands.

Formula

Resistance: R = (Vsupply − Vforward) ÷ I\nPower rating: P = I² × R (choose resistor with ≥2× rating for safety)\n\nTypical Vforward: Red=2.0V, Green=2.2V, Blue/White=3.0-3.3V\nTypical I: Standard indicator=20mA, high-brightness=30mA, low-current=2mA\n\nSeries LEDs: R = (Vs − n×Vf) ÷ I\nParallel LEDs: each LED gets its own resistor (don't share!)

Who Uses This Tool?

• Electronics hobbyists building LED circuits for projects
• Makers creating Arduino and Raspberry Pi LED lighting
• Automotive: adding LEDs to 12V vehicle electrical systems
• Repair technicians replacing burnt-out LEDs with correct current

Pro Tips

• Always measure actual Vf with a multimeter if possible — datasheet values are nominal and vary 5-10% between individual LEDs
• Never use a single resistor for parallel LEDs. Slight Vf differences cause one LED to hog current and fail, then cascade failure
• Choose a resistor with at least 2× the calculated power dissipation for reliability. Running a resistor near its max rating causes heat and premature failure
• If the calculated resistor value is close to 0 or negative, your supply voltage is too close to or below Vf. Use a higher supply voltage

Frequently Asked Questions about LED Resistor Calculator

What happens without a resistor?

The LED acts as a short circuit, draws unlimited current, and burns out in milliseconds. Always use a current-limiting resistor or constant-current driver.

Why can't I share a resistor between parallel LEDs?

LEDs have slightly different forward voltages. The one with the lowest Vf draws disproportionally more current, overheats, and fails. Each parallel LED needs its own resistor.

Free online LED Resistor Calculator — no signup, 100% client-side processing. All data stays in your browser.