Enter Values
Formula
R = (Vsupply - N × Vf) ÷ I
Presistor = (Vsupply - N × Vf) × I
Current must be converted from milliamps to amps before using the equations.
Example
One 2 V LED at 20 mA from a 12 V source requires at least 500 Ω. The resistor dissipates 0.2 W.
Selecting a Real Resistor
Choose the next higher standard resistance value to keep current at or below the target. Select a power rating comfortably above the calculated dissipation.
For the example, a 510 Ω resistor is a common practical choice, and a 0.5 W rating provides useful margin.
Important LED Behavior
Forward voltage varies by LED color, current, temperature, and part. Use the datasheet value at the intended operating current. Do not place LEDs directly in parallel behind one shared resistor because current may not divide evenly.
The supply voltage must remain higher than the total LED forward voltage with enough headroom for current regulation.