LED Series Resistor Calculator
The LED resistor calculator estimates a series resistor from supply voltage, LED forward voltage and target current, then suggests the next higher common E12 nominal value.
How to use this calculator
- Enter a supply voltage greater than the LED forward voltage and a positive target current in milliamps.
- Check the units and definitions before comparing results with another source.
- Change one input at a time when comparing scenarios, and keep the reporting period and assumptions consistent.
Formula used by the led resistor calculator
R = (Vsupply − Vforward) ÷ I; resistor power = voltage drop × current
- R = (Vsupply − Vforward) ÷ I; resistor power = voltage drop × current
- LED forward voltage and supply voltage vary. Verify the component data sheet, power rating and safe operating current; the standard series suggestion is not a circuit-safety certification.
- Inputs are checked for required values and valid ranges; displayed results are rounded for readability.
Worked examples
Worked example
Supply 5 V, LED forward voltage 2 V and target current 20 mA.
The ideal resistance is 3/0.02 = 150 Ω; resistor power is 0.06 W.
The ideal resistance is 3/0.02 = 150 Ω; resistor power is 0.06 W.
Understanding the result
The LED resistor calculator estimates a series resistor from supply voltage, LED forward voltage and target current, then suggests the next higher common E12 nominal value.
LED forward voltage and supply voltage vary. Verify the component data sheet, power rating and safe operating current; the standard series suggestion is not a circuit-safety certification.
Important notes
LED forward voltage and supply voltage vary. Verify the component data sheet, power rating and safe operating current; the standard series suggestion is not a circuit-safety certification.
The result applies to the numbers and assumptions entered here; verify specifications for safety-critical or regulated uses.
Frequently asked questions
Why use a resistor in series?
A series resistor limits current as the supply voltage exceeds the LED forward voltage.
Can I use the suggested resistor without checking power rating?
No. Verify power dissipation, tolerance, temperature and LED current limits before building a circuit.
How do I use this calculator?
Enter a supply voltage greater than the LED forward voltage and a positive target current in milliamps.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.