Last updated: 2026-06-16

LED Series Resistor

An LED needs a resistor in series with it to limit the current to a safe level. This calculator finds that resistor from three values: the supply voltage $V_S$, the LED's forward voltage $V_F$ and the forward current $I$ you want to run through it. It also reports the power the resistor must dissipate so you can choose one with the right rating.

Formula

The resistor drops the voltage left over after the LED takes its share, $V_S - V_F$. Dividing that by the chosen current gives the resistance from Ohm's law.

Worked example

A $9\ \text{V}$ supply drives a red LED with a $2\ \text{V}$ forward voltage at $20\ \text{mA}$.

The exact value 350 Ω is not a standard resistor, so the nearest higher standard value is 360 Ω; 330 Ω is also common and gives slightly more current. A $1/4\ \text{W}$ resistor easily covers the 0.14 W dissipation.

Resistor for common supplies

For a red LED ($V_F = 2\ \text{V}$) at $20\ \text{mA}$:

Why it matters

Because an LED has no internal current limiting, even a small excess voltage past its forward voltage can drive a damaging current. The series resistor sets a predictable operating point. When the computed resistance falls between standard values, pick the next higher standard value: it errs on the side of slightly less current, which keeps the LED within its rating. Leaving headroom on the power rating — typically choosing a resistor rated at twice the calculated dissipation — keeps the part running cool over time.