Calculate the series resistor needed to drive an LED at its rated current.
The resistor drops the difference between supply and the total LED forward voltage. Adequate headroom matters because LED forward voltage varies with temperature — with too little, current changes sharply. Driving LEDs with a resistor is only acceptable at low power; above about a watt a constant-current driver is needed for efficiency and stability.
LED Resistor
R = (V_supply − n V_f) ÷ I_LED
R = (V_supply − n V_f) ÷ I_LED The resistor drops the difference between supply and the total LED forward voltage. Adequate headroom matters because LED forward voltage varies with temperature — with too little, current changes sharply.
Driving LEDs with a resistor is only acceptable at low power; above about a watt a constant-current driver is needed for efficiency and stability.
This calculator takes 4 inputs: Supply voltage, LED forward voltage, Desired LED current, LEDs in series. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.