LED Resistor Calculator
Calculate current-limiting resistor values for LED circuits instantly
Circuit Parameters
Results will appear here
Enter values on the left
Quick Examples
What is an LED Resistor Calculator?
An LED resistor calculator helps you determine the correct current-limiting resistor value needed to safely operate an LED in a circuit. LEDs require a specific amount of current to function properly, and without proper current limiting, they can burn out instantly.
The calculator uses Ohm's Law to compute the resistor value based on your power supply voltage, LED forward voltage, and desired current. It also calculates the power dissipation to help you choose the appropriate resistor wattage rating.
How to Use This Calculator
Enter Source Voltage
Input the voltage of your power supply (battery, power adapter, or circuit voltage).
Enter LED Forward Voltage
Specify the LED's forward voltage drop (found in the LED datasheet or use presets).
Enter Desired Current
Input the LED current in milliamps (typically 20mA for standard LEDs).
Get Your Results
The calculator instantly shows the required resistor value, power dissipation, and nearest standard resistor values.
Understanding the Formula
Resistor Value (Ohm's Law)
The resistor value is calculated using: R = (Vs - Vled) / I
Where Vs is source voltage, Vled is LED forward voltage, and I is the desired current in amps.
Power Dissipation
Power dissipation is calculated using: P = (Vs - Vled) × I
This tells you how much power the resistor will dissipate as heat. Choose a resistor with wattage rating at least 2× the calculated power for safety.
Standard Resistor Values
Resistors come in standard values (E12, E24 series). The calculator finds the nearest standard value you can purchase. E24 series offers more precise values with 5% tolerance, while E12 series has 10% tolerance.
Common LED Forward Voltages
Red LED
Forward Voltage: 1.8-2.2V
Typical: 2.0V @ 20mA
Yellow LED
Forward Voltage: 1.9-2.2V
Typical: 2.1V @ 20mA
Green LED
Forward Voltage: 2.0-2.4V
Typical: 2.2V @ 20mA
Blue LED
Forward Voltage: 3.0-3.6V
Typical: 3.4V @ 20mA
White LED
Forward Voltage: 3.2-3.8V
Typical: 3.6V @ 20mA
UV/Purple LED
Forward Voltage: 3.0-4.0V
Typical: 3.4V @ 20mA
Frequently Asked Questions
Why do LEDs need resistors?
LEDs have very low internal resistance and will draw too much current if connected directly to a voltage source, which can destroy them instantly. A current-limiting resistor ensures the LED operates within its safe current range.
What happens if I use the wrong resistor value?
A resistor value that's too low can allow excessive current, damaging or destroying the LED. A value that's too high will reduce brightness or prevent the LED from lighting at all. Always use the calculated value or the nearest standard value above it for safety.
Can I use one resistor for multiple LEDs?
You can use one resistor for multiple LEDs if they're connected in series (add the forward voltages together). For parallel LEDs, each should have its own resistor to ensure equal current distribution and prevent one LED from hogging all the current.
What wattage resistor should I use?
Choose a resistor with at least double the calculated power dissipation for safety and longevity. For example, if the calculator shows 125mW power dissipation, use a 1/4W (250mW) resistor. The calculator recommends the appropriate wattage based on this safety factor.
Where do I find the LED forward voltage?
LED forward voltage is listed in the component's datasheet. If you don't have the datasheet, you can use the typical values provided by our color presets: red (~2V), green (~2.2V), blue (~3.4V), and white (~3.6V).