LED Resistor Calculator
Enter the supply voltage and the LED forward voltage and current from its datasheet. The calculator gives the exact resistance, the next standard resistor value and the power it must handle.
Result
- Next standard value (E24)
- 150 Ω
- Actual current with E12 value
- 20 mA
- Power dissipated in the resistor
- 60 mW
- Suggested resistor rating
- 0.125 W or higher
- Voltage across the resistor
- 3 V
The suggested rating is at least twice the dissipated power, which keeps the resistor cool. Rounding up to the next standard value lowers the current slightly and is safe for the LED.
Formula
- R = (supply voltage − LED forward voltage × number of LEDs) ÷ LED current
- Power in resistor = current² × R
How the LED Resistor Calculator works
An LED needs a resistor in series to limit its current. The resistor has to drop whatever voltage is left after the LED takes its forward voltage. Ohm's law then gives the resistance for the current you want.
Typical forward voltages are about 2 V for red and yellow, 2.2 V for green, and 3 to 3.3 V for blue and white. Most small indicator LEDs are rated for 20 mA.
Worked example
One red LED with a 2 V forward voltage at 20 mA on a 5 V supply: R = (5 − 2) ÷ 0.02 = 150 Ω. That is a standard value. It dissipates 0.02² × 150 = 0.06 W, so a 1/8 W resistor is enough.
Frequently asked questions
What resistor do I need for an LED on 12 V?
For a 2 V LED at 20 mA, (12 − 2) ÷ 0.02 = 500 Ω. The next standard value is 560 Ω. It dissipates about 0.18 W, so use a 1/2 W resistor.
Can I run several LEDs from one resistor?
Yes in series, as long as the supply voltage exceeds the sum of their forward voltages. Avoid one resistor for LEDs in parallel, because they will not share current evenly.
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