LED resistor calculator
Enter the supply voltage, the LED forward voltage and current, and how many LEDs are in series.
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How to use it
- Enter supply voltage, led forward voltage, led current and leds in series.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
R = (Vs − n × Vf) ÷ If, and resistor power P = (Vs − n × Vf) × I. The next higher E24 standard value is picked, so the current drops slightly.
Example
Inputs: Supply voltage = 5 V, LED forward voltage = 2 V, LED current = 20 mA, LEDs in series = 1. Result: Calculated resistance: 150 Ω; Next standard value (E24): 150 Ω; Current with standard value: 20 mA.
Limits and caveats
- Forward voltage varies with color and maker (red about 2 V, white and blue about 3 V); take it from the datasheet.
- Pick a resistor rated at least twice the calculated power so it stays cool.
- Not for high-power LEDs, which need a constant-current driver.
Questions about this tool
What resistor for a red LED on 5 V?
With a 2 V forward voltage and 20 mA you need 150 Ω, which is a standard value.
Why not connect an LED straight to the supply?
LED current rises very steeply with a small voltage increase, so without a limiting resistor it burns out.
Can one resistor serve several LEDs in parallel?
Not recommended: small forward-voltage differences make the current crowd into one LED. Give each LED its own resistor or wire them in series.