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LED Series Resistor Calculator

Calculate the series resistor any LED needs from supply voltage, forward voltage and current, with the nearest standard value.

LED resistor

Enter the LED specs. Results update live.

Resistor needed
-

Enter the LED specs to size the resistor.

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The LED resistor formula

The resistor drops the voltage the LED does not need: supply voltage minus LED forward voltage, divided by the LED current. A 9 volt supply with a 2.1 volt LED at 20 milliamps needs (9 - 2.1) / 0.02 = 345 ohms. You then pick the nearest standard E12 value at or above that, which is 390 ohms.

Why the wattage rating matters

The resistor turns the dropped voltage into heat, and an undersized resistor overheats and fails. The safe practice is a power rating at least double the dissipated power, so a resistor burning 0.14 watts should be rated 0.5 watts.

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LED Resistor Calc FAQs

What resistor do I need for a 5V LED circuit?
For a typical red LED (2V forward, 20mA) on 5V: (5-2)/0.02 = 150 ohms. Enter your exact LED specs above for the precise value and wattage.
What is the forward voltage of common LEDs?
Red and yellow are about 1.8 to 2.2V, green about 2.2 to 3V, blue and white about 3 to 3.4V. Check your LED datasheet for the exact number.
Can I use a larger resistor than calculated?
Yes, a larger resistor just makes the LED dimmer. Going one E12 step up is normal and safe; going far larger may make the LED too dim to see.
Do I need a resistor for each LED?
Yes, unless LEDs are wired in series sharing one resistor. Parallel LEDs without individual resistors will not share current evenly and can burn out.
What happens if I skip the resistor?
The LED will draw uncontrolled current and burn out in seconds, and may damage the power supply. Always use a current limiting resistor.
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