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LED Resistor Calculator — Current-Limiting Resistor From V & mA

Enter your supply voltage, the LED's forward voltage (check the datasheet, or use a common default) and target current to calculate the current-limiting resistor value needed — skipping this resistor is the single most common reason a first LED circuit burns out instantly.

How to use LED Resistor

  1. Enter supply voltage.
  2. Enter the LED's forward voltage.
  3. Enter target current (typically 15-20mA for a standard LED).
  4. Read the calculated resistor value.

Frequently asked questions

Why does an LED need a resistor at all?
An LED has very low internal resistance once forward voltage is exceeded, so without a limiting resistor, current draw is limited only by the power supply and the LED's internal resistance — which usually means well past its rated current, damaging it almost immediately.
What forward voltage should I use if I don't know the LED's exact spec?
Roughly 2V for standard red/yellow LEDs and roughly 3-3.4V for blue/white/UV LEDs are common defaults, though checking the actual datasheet gives a more accurate result.
What target current should I aim for?
15-20mA is a safe, bright default for most standard 5mm LEDs; check the datasheet's maximum forward current rating for anything unusual, like a high-brightness or SMD LED.

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