LED Resistor Calculator
Size the series resistor an LED needs from your supply voltage, its forward voltage, and current.
How it works
An LED isn't a resistor — connect it straight across a battery and it'll draw far too much current and burn out. You add a resistor in series to soak up the leftover voltage and hold the current at a safe level.
The math is Ohm's law. Take your supply voltage, subtract the LED's forward voltage drop, and divide what's left by the current you want in amps. A 5-volt supply, a 2-volt LED, and 20 milliamps gives (5 − 2) ÷ 0.02 = 150 ohms.
Real resistors only come in set values, so the tool also snaps the answer to the nearest E12 standard part and shows how much power the resistor will dissipate — handy for checking a tiny quarter-watt resistor won't cook.
Frequently asked questions
What forward voltage should I use?
It depends on color: red LEDs sit around 1.8–2.2 V, while blue and white run closer to 3–3.4 V. Check the datasheet if you have it, or start with a typical value for the color.
How much current does an LED need?
Most standard indicator LEDs are happy at 10–20 mA. Brighter isn't always better — 20 mA is a common safe maximum, and many LEDs look plenty bright at half that.
Why pick the nearest standard value?
You can't buy a 147-ohm resistor off the shelf, but 150 ohms is everywhere. Rounding to the nearest E12 value changes the current only slightly and keeps the design buildable.