To connect an LED directly in a circuit, it needs to have a current limiting resistor in series with it. Otherwise the LED will receive too much current and will fail. Use this handy LED resistor calculator to select the correct resistance for your LED circuit!

LED Resistor Calculator

Enter the voltage, forward voltage, and current ratings for your LED(s) below:


Volt (Typical value: 2v)
Milliamps (Typical value: 20mA)



LEDs are designed to operate under ideal conditions. They generally have a specified input voltage, forward voltage, and a maximum current rating that needs to be adhered to when designing a circuit.  To limit the current in an LED circuit requires a resistor unless the LED is in series with some other component that is limiting current.  This resistor will always be placed in series with the LED.

LED resistor calculator

The forward voltage (or voltage drop) across an LED varies by LED and manufacturer. Usually, it is directly tied to the color of the LED due to the different materials used in the manufacturing process.  The most common voltage drop is 2 volts, but you should always check the manufacturer’s data sheet before building your circuit or sizing the resistor!

Typical Forward Voltages (Vf) are:

  • Red: 2
  • Green: 2.1
  • Blue: 3.6
  • White: 3.6
  • Yellow: 2.1
  • Orange: 2.2
  • Amber: 2.1
  • Infrared: 1.7

LED Resistor Calculator Formula

The formula for the LED resistor calculator is pretty simple.  To determine the correct resistance the formula is: R = (Vs – Vf) * N / If

  • Vs = This is your voltage source.
  • Vf = The LED’s rated Forward Voltage
  • If = The LED’s maximum current rating.
  • N = The number of LEDs (in series) in the circuit.

You may also be interested in our tutorial on resistance! We also have a resistor calculator to help you figure out the color bands.