Use the calculator below to determine the series resistor needed to connect various series combinations of light emitting diodes or "LEDs". LED voltages vary from about 1.6 to 4.5 with 2 volts being a close approximation for red and orange LEDs. Blue and white LEDs will require higher voltages with blue being about 3.5 volts and white about 4.5 volts.To find the series resistor needed, enter the total number of LEDs, the individual LED voltage (1.6 to 4.5), the desired current (normally 10 to 20 mA) and the total supply voltage (3,6,9,12, etc.) Press "find resistance" to find the resistor value needed.

Use a resistor rated higher than the displayed power in the box below "Series Resistor", most cases can use a 1/4 watt resistor. Several common LED connections are shown in the drawing below the calculator.

For example, the fourth illustration down shows 4 LEDs connected to a 12 volt source which requires a 270 ohm resistor. The examples assume a LED voltage of 2 volts and a current of 15 mA.