Category: Calculators
LED Resistor Calculator
Series resistor value and power rating for an LED
Enter your supply voltage and an LED's forward voltage and current to calculate the series resistor value and power rating needed to drive it safely.
Series resistor350 Ω
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What this tool does
An LED needs a resistor in series to limit current to its rated forward current; without one, an LED draws far more current than it can handle and burns out almost immediately. The resistor value is the leftover voltage divided by the target current: R = (Vsupply − Vled) / Iled.
How to use it
- Enter your supply voltage.
- Enter the LED's forward voltage and forward current from its datasheet.
- Set how many identical LEDs are wired in series under one resistor, if more than one.
- Read the resistor value and its minimum power rating underneath.
Privacy
This tool runs entirely in your browser. Your input is never uploaded, stored or shared — closing the tab removes it.
Frequently asked questions
- Where do I find an LED's forward voltage and current?
- Both are in the LED's datasheet. As rough defaults, standard red or yellow LEDs are commonly around 2 V at 20 mA, and blue or white LEDs are commonly around 3 to 3.2 V at 20 mA, but always check the actual part.
- What resistor power rating should I buy?
- Buy a resistor rated for at least double the calculated power, rounded up to a common size (an eighth, quarter or half watt). This calculator shows the exact dissipated power; real resistors are sold in standard power ratings, and derating extends their life.
- Can I wire several LEDs under one resistor?
- Only if they are identical and wired in series, sharing one resistor sized for the combined forward voltage. Never wire LEDs in parallel under a single resistor: the slightly weaker LED hogs more current, heats up, and the imbalance compounds until it fails.