Find the series resistor an LED needs to run safely, plus the wattage that resistor has to handle.
How to Use the LED Resistor Calculator
Enter your supply voltage and target forward current, then pick a preset LED type or choose Custom to type in the exact forward voltage from a datasheet. Resistance and power dissipation both recalculate as soon as any field changes.
An LED isn't a resistor — feed it more current than it's rated for and it doesn't glow brighter forever, it overheats and dies, often within seconds on a fresh 9V battery with no resistor at all. The series resistor's entire job is to eat the voltage the LED itself doesn't need and cap the current at a safe value. Two formulas cover the whole calculation:
R = (Vsupply − Vf) ÷ If
P = If² × RVf is the LED's own forward voltage — the amount it drops regardless of what's feeding it — and If is the current you want flowing through it, typically around 20 mA for a standard indicator LED. Subtract Vf from the supply, divide by the target current, and what's left is the resistor value that makes the arithmetic balance.
What Happens Without a Resistor?
An LED's forward voltage barely moves once it's conducting — push more current through it and the voltage across it stays nearly flat while current climbs steeply, which is exactly the opposite of how a resistor behaves. Wire an LED straight across a battery with nothing in series and there's nothing to limit that current climb except the LED's own internal resistance and the wiring, both usually too small to save it.
Forward voltage isn't one number across all LEDs, either — it shifts with color because it's tied to the semiconductor's band gap. A standard red LED sits around 2.0V, green and yellow LEDs run a bit higher near 2.1V, and blue or white LEDs need closer to 3.0–3.4V before they'll conduct at all. Using a red LED's typical voltage for a white LED will undersize the resistor and overdrive the current, so match the preset — or the datasheet — to the actual part.
The power figure matters just as much as the resistance. A resistor rated below its calculated dissipation will run hot and can fail outright; the safe move is picking the next standard wattage up from whatever this calculator reports — if it comes out to 0.18W, reach for a 1/4W (0.25W) resistor rather than an eighth-watt part sitting right at the edge.