I usually use this method: (derived from R=E/I) R=(V-Vf)/I R=Resistor Needed V=Voltage applied to the resistor and LED Vf is the forward voltage of the LED. I is the current used by the LED (typically 20mA for red.) Remember, I is in Amps, so you must convert the LED's req'd mA into A. An LED is a current device, not a voltage device. It will always drop the voltage by Vf. Adding more current will make them brighter.
