To calculate the resistance when powering a LED use this: R=(Vpow-Vled)/I R: is the needed resistance Vpow: is the voltage of our source or power supply (e.g. battery) Vled: voltage of the LED I: the maximum of current that goes through our LED. for a 2V (diode forward voltage) LED we need 20mA =0.02A (diode forward current) So,