LED Resistor Calculator
Calculate the correct current-limiting resistor value for your LED circuit. Get power dissipation and nearest standard resistor values.
Calculator
Common LED Values
How to Use This Calculator
This LED resistor calculator helps you find the correct current-limiting resistor for your LED circuit. Simply enter your power supply voltage, LED forward voltage, and desired LED current to get the resistor value.
- Supply Voltage (Vs) — Enter your power supply voltage (e.g., 5V, 12V)
- LED Forward Voltage (Vf) — Enter the LED's forward voltage from its datasheet, or click a common LED color below the calculator
- LED Current (If) — Enter the desired LED current (typically 10-20mA for standard LEDs)
- Click "Calculate" to see the required resistor value and nearest standard E24 value
The LED Resistor Formula
The resistor value is calculated using Ohm's Law. The resistor must drop the difference between the supply voltage and the LED forward voltage.
Power Dissipation
The resistor dissipates power as heat. Calculate it to ensure your resistor can handle the load:
Choose a resistor rated for at least 2x the calculated power dissipation. For most LED circuits with 20mA, a standard 1/4W resistor is sufficient.
LED Specifications by Color
Different LED colors have different forward voltage drops due to the semiconductor materials used. Here are typical values:
| LED Color | Forward Voltage (Vf) | Typical Current |
|---|---|---|
| Red | 1.8 - 2.2V | 20mA |
| Orange | 2.0 - 2.2V | 20mA |
| Yellow | 2.0 - 2.2V | 20mA |
| Green | 2.0 - 3.5V | 20mA |
| Blue | 3.0 - 3.5V | 20mA |
| White | 3.0 - 3.5V | 20mA |
| Infrared | 1.2 - 1.6V | 20-50mA |
| UV | 3.0 - 4.0V | 20mA |
Note: Always check your LED's datasheet for exact specifications. High-brightness LEDs may have different voltage and current requirements.
Practical Examples
Example 1: Red LED on 5V Arduino
Power a red LED from an Arduino's 5V output at 20mA.
Given: Vs = 5V, Vf = 2.0V, If = 20mA
Formula: R = (5V - 2V) / 0.020A = 150Ω
Result: Use a 150Ω resistor (E24 standard value)
Example 2: Blue LED on 12V Supply
Power a blue LED from a 12V power supply at 20mA.
Given: Vs = 12V, Vf = 3.2V, If = 20mA
Formula: R = (12V - 3.2V) / 0.020A = 440Ω
Result: Use a 430Ω resistor (nearest E24 value)
Example 3: White LED at Reduced Brightness
Run a white LED dimmer at 10mA from 3.3V supply.
Given: Vs = 3.3V, Vf = 3.0V, If = 10mA
Formula: R = (3.3V - 3.0V) / 0.010A = 30Ω
Result: Use a 33Ω resistor (nearest E24 value)
Frequently Asked Questions
Why do LEDs need a current-limiting resistor?
LEDs have very low internal resistance. Without a current-limiting resistor, the LED would draw excessive current and burn out instantly. The resistor limits the current to a safe level specified by the LED's datasheet.
What happens if I use a smaller resistor?
A smaller resistor allows more current, making the LED brighter but potentially shortening its lifespan or causing immediate failure. Stay within the LED's maximum current rating (typically 20-30mA for standard LEDs).
What happens if I use a larger resistor?
A larger resistor reduces current, making the LED dimmer. This is safe and can extend LED life, but the LED may be too dim for your application.
Can I connect multiple LEDs with one resistor?
For LEDs in series: Yes, add up the forward voltages and use the formula with the total Vf. For LEDs in parallel: No, each LED needs its own resistor to ensure equal current distribution.
What resistor power rating do I need?
Calculate power using P = (Vs - Vf) × If. For a 5V supply with a red LED (2V) at 20mA: P = 3V × 0.02A = 60mW. A standard 1/4W (250mW) resistor is sufficient with plenty of margin.
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