LED Resistor Calculator

In short An LED needs a series resistor because it is a diode, not a resistor. Past its forward voltage the current rises almost vertically for a tiny increase in voltage, so without something to limit it the current is decided by whatever the supply and the wiring happen to allow — and the LED destroys itself. The resistor is what makes the current predictable. The arithmetic is one subtraction and one division: take the LED's forward voltage off the supply, then divide what is left by the current you want. A 2 V red LED at 20 mA on a 5 V supply leaves 3 V across the resistor, and 3 ÷ 0.02 is 150 Ω — which happens to be a real E24 value, so no rounding is needed. When the exact figure is not a stock value, round UP: a larger resistor means slightly less current and a slightly dimmer LED, while a smaller one means more current than the part is rated for. Two rules follow from the same physics. Never put several LEDs in parallel behind one resistor — their forward voltages differ even between parts from the same reel, so the lowest takes most of the current, gets hotter, drops further and takes more still. And LEDs in series all share one current, so one resistor is correct for the string, provided the supply exceeds the sum of their forward voltages.