To calculate the resistor needed for a simple LED circuit, simply take the voltage drop away from the source voltage then apply Ohm's Law. In other words...

where:

- V
_{S}is the source voltage, measured in volts (V), - V
_{LED}is the voltage drop across the LED, measured in volts (V), - I
_{LED}is the current through the LED^{*}, measured in Amperes (Amps/A), and - R is the resistance, measured in Ohms (Ω).

** The current through the circuit is constant so I*

_{LED}is also the current through the resistor.This calculator is based on the Ohms Law Calculator, but takes into consideration the voltage drop from the LED. To use the calculator, enter any three known values and press "Calculate" to solve for the others.