LED Resistor Calculator - Series Resistor Value, Power & Standard Pick
LED Resistor Calculator
Pick the right resistor so your LED lives a long life
The Right Resistor
R = (supply − LED voltage) ÷ current, with several LEDs in series supported.
Rounded Up Safely
The standard value is never below the calculated one, so the LED current never exceeds your target.
Power Rating
See the heat the resistor must handle and a suggested rating with a 2× safety margin.
Full Privacy
Everything is calculated in your browser.
Why an LED Needs a Resistor
An LED drops a nearly fixed forward voltage and then lets current rise very steeply, so connecting it straight to a supply can destroy it in an instant. A series resistor takes up the difference: for a 5 V supply, a red LED at 2 V and 20 mA needs (5 − 2) ÷ 0.02 = 150 Ω, and the resistor dissipates 3 V × 20 mA = 60 mW, well within a common ¼ W part.
Forward voltages here are typical figures and real LEDs differ, so use your datasheet when you can. With several LEDs in series their voltages add, and the supply must exceed the total. Rounding the resistor up slightly lowers the current and brightness, which usually extends the LED's life; do not round down.
Key Takeaways
- Series LEDs: Sum the forward voltages of up to 50 LEDs.
- Colour presets: Typical forward voltages for common colours, editable.
- Safety checks: Warns when the supply is too low for the LED chain.