LED Resistor Calculator

Find the current-limiting resistor for one LED, a series string, or a parallel array. Exact value, wattage, and nearest standard part. Runs locally in your browser.

Supply voltage (Vs)

volts (V)

LED forward voltage (Vf)

volts (red ~2, blue/white ~3.2)

LED current (If)

milliamps (mA), per LED

Number of LEDs

in series

Standard series

suggested nearest resistor

Enter the values to compute the resistor.

The resistor drops the surplus voltage and sets the current: R = (Vs − N×Vf) ÷ If. For a series string the forward voltages add (one current path), so the supply must exceed N×Vf or it won't light. For a parallel array the currents add and every LED still sees only Vf — a single shared resistor then balances them, which is fine for matched LEDs but in practice each LED should get its own resistor so a warmer LED doesn't hog current. Pick a resistor rated for at least 2× the computed power for headroom, and round resistance up to lower the current if in doubt. Runs locally — nothing leaves your browser.