LED Resistor Calculator

LED Resistor Calculator

Supply Voltage:
Forward Voltage:
Forward Current (Amps - not mA):
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LED Resistor Formula:

R = (Vs - Vf) / If

Resistor = (Supply Voltage - Forward Voltage)/ Forward Current

* Note: Forward Current is in Amps, not Milliamps (30mA = .02A)

LED Resistor Definition

This easy-to-use calculator instantly tells you the proper size resistor to use for your particular LED’s characteristics and your power supply voltage. Choosing the correct value of the resistor is essential to proper operation of the LED.

LEDs, or Light-Emitting-Diodes, are semiconductor devices that emit light of various colors depending on the substrate used between the P and N layers. They are used in applications such as indicator lights and infrared remote controls. In the last decade, affordable higher wattage LEDs are replacing fluorescent and incandescent lighting systems.

Electrically, LEDs operate like other diodes. They act like a switch, allowing current to flow when their threshold voltage is applied. The amount of current increases exponentially over a narrow voltage range. Thus, it is important to keep the current constant by regulating the voltage. That’s the job of the resistor and why its value is crucial.

To use this calculator you need two values from the LED specification and the voltage supplied by your power source:

  1. The forward voltage of the LED, Vf
  2. The forward current of the LED, If
  3. The supply voltage, Vs

Forward Voltage Vf

Vf comes from the data sheet of your LED and indicates the threshold value at which the LED turns on. The data sheet also lists the maximum value of Vf. Typically, there is less than a volt difference between the two.

Forward Current If

This value is specified as a maximum current rating. If it’s exceeded the LED may fail.

Supply Voltage Vs

The power supply voltage, Vs, is the amount of voltage coming into the resistor. It is always greater than Vf. For instance, if you power your LED with four AA alkaline batteries, then your supply voltage is 4 x 1.5V = 6V.

An Example

Let’s say that you want to take advantage of one of the newer high wattage LEDs in a circuit, one rated at 3 watts. The data sheet for such an LED will typically give values for such a device as:

Vf – 2.95-3.25V
If – max. 700 mA

If you use a 12 volt power supply for the circuit then Vs is 12V.

To find the proper size resistor for this LED, in the calculator fields type:

  • Supply voltage: 12
  • Forward voltage: 2.95
  • Forward current: .7 (Note: this field’s units is Amps, not mA. A current expressed as mA must first be divided by 1000).

Press Calculate and the calculator will yield the result of 12.9 Ohms. Pick a resistor value that’s within 10% of this value. Also be sure to check out the LED Lighting Calculator.


*Tip: Be sure that the wattage rating of the resistor is sufficient (greater than Vf x If). Also, be sure that your voltage supply provides enough current.

How to Calculate LED Resistor

Let's be honest - sometimes the best led resistor calculator is the one that is easy to use and doesn't require us to even know what the led resistor formula is in the first place! But if you want to know the exact formula for calculating led resistor then please check out the "Formula" box above.

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