LED Series Resistor Calculator

Series resistor for an LED string — value, nearest E-series part and power.

Supply voltage Vs (V)

LED forward voltage Vf (V)

LED current If

LEDs in series

—

Series resistor value

Sizing the resistor

The resistor takes whatever supply voltage the LEDs don't, at the design current:

R = \frac{V _{s} - n V _{f}}{I _{f}} P_{R} = (V_{s} - n V_{f}) I_{f}

with $n$ LEDs in series. Pick the next standard E-series value above the result so you never exceed the rated current, and choose a resistor wattage at least 2× the calculated $P_{R}$ for margin. If $V_{s} < n V_{f}$ there isn't enough headroom to light the string — drop an LED or raise the supply.

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