VS = volts
Vf = volts
I  = milliamps

Enter the values above, and click calculate.

In order to calculate the size of a resistor needed to limit the current through an LED, there are a few things that you need to know first. Those are:

  1. What source voltage (VS) you will be using, such as a 9V battery, or 12V if connecting to a car battery.
  2. What is the forward drop voltage (Vf). All LED's have a forward drop voltage. This is the voltage where the LED will first start to conduct. Anything lower, the LED is completely off, and draws no current at all. A typical forward drop voltage would be 2.0V.
  3. The current value (I) that your LED will run at, typically not the maximum, enough to make your LED as bright as you like, without burning it out.

Once you know these, the next thing is to apply this formula.

R = (VS - Vf)/I

Free JavaScripts provided
by The JavaScript Source