LED Resistor Calculator
Find the series resistor for an LED from supply, forward voltage, and current.
Size an LED’s series resistor
An LED needs a series resistor to limit its current, and the right value comes straight from Ohm’s law. Enter your supply voltage, the LED’s forward voltage, and the target current, and this returns the resistor to use.
For a 5 V supply, a 2 V LED, and 20 mA, the resistor must drop 3 V at 0.02 A, which is 150 Ω dissipating 0.06 W. The tool also flags the nearest standard resistor value.
Practical and private
It applies the exact voltage-drop calculation and reports the power the resistor handles, all in your browser with nothing uploaded.
Frequently Asked Questions
How is the resistor value found?
By Ohm’s law on the voltage the resistor must drop: R = (supply − LED forward voltage) / current. The tool also picks the nearest standard value.
Why does the LED need a resistor?
LEDs have almost no internal resistance, so without a series resistor to limit current they draw too much and burn out.
What current should I use?
Many indicator LEDs run at about 20 mA (0.02 A), but check the LED’s datasheet for its rated forward current.
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