LED Resistor Calculator

Work out the series resistor an LED needs from supply voltage, forward voltage, and forward current — plus the power the resistor dissipates. Calculates in your browser.

Series resistor

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Nearest standard (E12)-
Resistor power-

Show the math

The resistor drops the leftover voltage at the LED's current:

R = (Vsupply − Vforward) ÷ I

Pick a resistor rated above the shown power. Typical forward voltages: ~2 V red, ~3.1 V blue/white. All math runs in your browser.

What this does

An LED resistor calculator works out the series resistor an LED needs from supply voltage, forward voltage, and forward current, plus the power the resistor dissipates.

How to use it

  1. Enter the supply voltage.
  2. Enter the LED forward voltage.
  3. Enter the forward current.
  4. Read the resistor and its wattage.

How it works

The resistor drops the excess voltage at the chosen current. Pick a resistor rated comfortably above the dissipated power, typically at least double.

R = (Vsupply − Vf) ÷ I; P = (Vsupply − Vf) × I

Understanding your result

The resistor exists to drop the excess voltage and hold the current safe, because an LED has no resistance of its own and will destroy itself without it. Pick the next standard resistor value above the calculated one to stay on the safe side, and rate its wattage well above the figure shown, typically at least double, so it runs cool rather than scorching.

Example

A 2 V, 20 mA LED on 5 V needs about a 150 Ω resistor.

Sources & methodology

Last updated .

Frequently asked questions

How is the LED resistor calculated?

Subtract the LED’s forward voltage from the supply, then divide by the desired current: R = (Vsupply − Vf) ÷ I.

What resistor wattage do I need?

The resistor dissipates P = (Vsupply − Vf) × I. Pick a resistor rated comfortably above that, typically at least double.

Is my data stored?

No. Everything is calculated locally in your browser.